Add -f[no-]warn-unsupported-llvm-version. Closes Trac #7579.

This controls whether or not the compiler warns if we're using an LLVM
version that's too old or too new. It's mostly useful when building the
compiler knowingly with an unsupported version, so you don't get a lot
of warnings in the build process.

There's no documentation for this since it's a flag only a few
developers would care about anyway.
Signed-off-by: Austin Seipp <mad.one@gmail.com>

compiler/llvmGen/LlvmCodeGen.hs

@@ -68,11 +68,12 @@ llvmCodeGen dflags h us cmms
         ver <- (fromMaybe defaultLlvmVersion) `fmap` figureLlvmVersion dflags
         -- cache llvm version for later use
         writeIORef (llvmVersion dflags) ver
-        when (ver < minSupportLlvmVersion) $
+        let doWarn = wopt Opt_WarnUnsupportedLlvmVersion dflags
+        when (ver < minSupportLlvmVersion && doWarn) $
             errorMsg dflags (text "You are using an old version of LLVM that"
                              <> text " isn't supported anymore!"
                              $+$ text "We will try though...")
-        when (ver > maxSupportLlvmVersion) $
+        when (ver > maxSupportLlvmVersion && doWarn) $
             putMsg dflags (text "You are using a new version of LLVM that"
                            <> text " hasn't been tested yet!"
                            $+$ text "We will try though...")

compiler/main/DynFlags.hs

@@ -425,6 +425,7 @@ data WarningFlag =
    | Opt_WarnSafe
    | Opt_WarnPointlessPragmas
    | Opt_WarnUnsupportedCallingConventions
+   | Opt_WarnUnsupportedLlvmVersion
    | Opt_WarnInlineRuleShadowing
    deriving (Eq, Show, Enum)
 
@@ -2398,7 +2399,8 @@ fWarningFlags = [
   ( "warn-safe",                        Opt_WarnSafe, setWarnSafe ),
   ( "warn-pointless-pragmas",           Opt_WarnPointlessPragmas, nop ),
   ( "warn-unsupported-calling-conventions", Opt_WarnUnsupportedCallingConventions, nop ),
-  ( "warn-inline-rule-shadowing",       Opt_WarnInlineRuleShadowing, nop ) ]
+  ( "warn-inline-rule-shadowing",       Opt_WarnInlineRuleShadowing, nop ),
+  ( "warn-unsupported-llvm-version",    Opt_WarnUnsupportedLlvmVersion, nop ) ]
 
 -- | These @-\<blah\>@ flags can all be reversed with @-no-\<blah\>@
 negatableFlags :: [FlagSpec GeneralFlag]
@@ -2783,6 +2785,7 @@ standardWarnings
         Opt_WarnAlternativeLayoutRuleTransitional,
         Opt_WarnPointlessPragmas,
         Opt_WarnUnsupportedCallingConventions,
+        Opt_WarnUnsupportedLlvmVersion,
         Opt_WarnInlineRuleShadowing,
         Opt_WarnDuplicateConstraints
       ]