Make Rank2Types and PolymorphicComponents into a synonym for RankNTypes (no deprecation)

There was a long thread on cvs-ghc which concluded that although it's
the Right Thing to deprecate Rank2Types and PolymorphicComponents, in
favour of RankNTypes, it would cause lots of busy-work for library
authors and the pain isn't worth the gain.

So this patch removes the deprecation, and documents the synonym-ity.

compiler/main/DynFlags.hs

@@ -2457,11 +2457,11 @@ xFlags = [
   ( "CApiFFI",                          Opt_CApiFFI, nop ),
   ( "GHCForeignImportPrim",             Opt_GHCForeignImportPrim, nop ),
   ( "LiberalTypeSynonyms",              Opt_LiberalTypeSynonyms, nop ),
-  ( "PolymorphicComponents",            Opt_RankNTypes, 
-     deprecatedForExtension "RankNTypes" ),
-  ( "Rank2Types",                       Opt_RankNTypes, 
-     deprecatedForExtension "RankNTypes" ),
+
+  ( "PolymorphicComponents",            Opt_RankNTypes, nop),
+  ( "Rank2Types",                       Opt_RankNTypes, nop),
   ( "RankNTypes",                       Opt_RankNTypes, nop ),
+
   ( "ImpredicativeTypes",               Opt_ImpredicativeTypes, nop),
   ( "TypeOperators",                    Opt_TypeOperators, nop ),
   ( "ExplicitNamespaces",               Opt_ExplicitNamespaces, nop ),

docs/users_guide/flags.xml

@@ -914,9 +914,7 @@
             <entry>Enable <link linkend="explicit-foralls">explicit universal quantification</link>.
               Implied by <option>-XScopedTypeVariables</option>,
               <option>-XLiberalTypeSynonyms</option>,
-              <option>-XRank2Types</option>,
               <option>-XRankNTypes</option>,
-              <option>-XPolymorphicComponents</option>,
               <option>-XExistentialQuantification</option>
             </entry>
             <entry>dynamic</entry>
@@ -925,13 +923,13 @@
           <row>
             <entry><option>-XPolymorphicComponents</option></entry>
             <entry>Enable <link linkend="universal-quantification">polymorphic components for data constructors</link>.</entry>
-            <entry>dynamic</entry>
+            <entry>dynamic, synonym for <option>-XRankNTypes</option></entry>
             <entry><option>-XNoPolymorphicComponents</option></entry>
           </row>
           <row>
             <entry><option>-XRank2Types</option></entry>
             <entry>Enable <link linkend="universal-quantification">rank-2 types</link>.</entry>
-            <entry>dynamic</entry>
+            <entry>dynamic, synonym for <option>-XRankNTypes</option></entry>
             <entry><option>-XNoRank2Types</option></entry>
           </row>
           <row>

docs/users_guide/glasgow_exts.xml

@@ -2071,9 +2071,7 @@ The following syntax is stolen:
 	Stolen (in types) by: <option>-XExplicitForAll</option>, and hence by
             <option>-XScopedTypeVariables</option>,
 	    <option>-XLiberalTypeSynonyms</option>,
-	    <option>-XRank2Types</option>,
 	    <option>-XRankNTypes</option>,
-	    <option>-XPolymorphicComponents</option>,
 	    <option>-XExistentialQuantification</option>
 	  </para></listitem>
       </varlistentry>
@@ -6384,20 +6382,12 @@ The function <literal>f3</literal> has a rank-3 type;
 it has rank-2 types on the left of a function arrow.
 </para>
 <para>
-GHC has three flags to control higher-rank types.  (All three imply <option>-XExplicitForAll</option>, <xref linkend="explicit-foralls"/>.)
-<itemizedlist>
-<listitem><para>
- <option>-XPolymorphicComponents</option>: data constructors (only) can have polymorphic argument types.
-</para></listitem>
-<listitem><para>
- <option>-XRank2Types</option>: any function (including data constructors) can have a rank-2 type.
-</para></listitem>
-<listitem><para>
- <option>-XRankNTypes</option>: any function (including data constructors) can have an arbitrary-rank type.
-That is,  you can nest <literal>forall</literal>s
+The language option <option>-XRankNTypes</option> (which implies <option>-XExplicitForAll</option>, <xref linkend="explicit-foralls"/>)
+enables higher-rank types.
+That is, you can nest <literal>forall</literal>s
 arbitrarily deep in function arrows.
-In particular, a forall-type (also called a "type scheme"),
-including an operational type class context, is legal:
+For example, a forall-type (also called a "type scheme"),
+including a type-class context, is legal:
 <itemizedlist>
 <listitem> <para> On the left or right (see <literal>f4</literal>, for example)
 of a function arrow </para> </listitem>
@@ -6407,15 +6397,20 @@ field type signatures.</para> </listitem>
 <listitem> <para> As the type of an implicit parameter </para> </listitem>
 <listitem> <para> In a pattern type signature (see <xref linkend="scoped-type-variables"/>) </para> </listitem>
 </itemizedlist>
-</para></listitem>
-</itemizedlist>
-Either <option>-XRank2Types</option> or <option>-XRankNTypes</option> is also required for any 
+The <option>-XRankNTypes</option> option is also required for any 
 type with a <literal>forall</literal> or
 context to the right of an arrow (e.g. <literal>f :: Int -> forall a. a->a</literal>, or
 <literal>g :: Int -> Ord a => a -> a</literal>).  Such types are technically rank 1, but
 are clearly not Haskell-98, and an extra flag did not seem worth the bother.
 </para>
 
+<para>
+The obselete language options <option>-XPolymorphicComponents</option> and <option>-XRank2Types</option>
+are synonyms for <option>-XRankNTypes</option>.  They used to specify finer distinctions that
+GHC no longer makes.  (They should really elicit a deprecation warning, but they don't, purely 
+to avoid the need to library authors to change their old flags specifciations.)
+</para>
+
 <sect3 id="univ">
 <title>Examples
 </title>