Summary:
Certain syntactic elements have integers in them, such as fixity
specifications, SPECIALISE pragmas and so on.

The lexer will accept mult-radix literals, with arbitrary leading zeros
in these.

Bring in a SourceText field to each affected AST element to capture the
original literal text for use with API Annotations.

Affected hsSyn elements are

```
     -- See note [Pragma source text]
     data Activation = NeverActive
                     | AlwaysActive
                     | ActiveBefore SourceText PhaseNum
                          -- Active only *strictly before* this phase
                     | ActiveAfter SourceText PhaseNum
                           -- Active in this phase and later
                     deriving( Eq, Data, Typeable )
                               -- Eq used in comparing rules in HsDecls

     data Fixity = Fixity SourceText Int FixityDirection
       -- Note [Pragma source text]
       deriving (Data, Typeable)
 ```

and

```
      | HsTickPragma         -- A pragma introduced tick
         SourceText           -- Note [Pragma source text] in BasicTypes
         (StringLiteral,(Int,Int),(Int,Int))
                                          -- external span for this tick
         ((SourceText,SourceText),(SourceText,SourceText))
            -- Source text for the four integers used in the span.
            -- See note [Pragma source text] in BasicTypes
         (LHsExpr id)
```

Updates haddock submodule

Test Plan: ./validate

Reviewers: goldfire, bgamari, austin

Reviewed By: bgamari

Subscribers: thomie, mpickering

Differential Revision: https://phabricator.haskell.org/D1781

GHC Trac Issues: #11430