Add infix flag for class and data declarations

At the moment, data and type declarations using infix formatting produce the same AST as those using prefix.

So

type a ++ b = c

and

type (++) a b = c

cannot be distinguished in the parsed source, without looking at the OccName details of the constructor being defined.

Having access to the OccName requires an additional constraint which explodes out over the entire AST because of its recursive definitions.

In keeping with moving the parsed source to more directly reflect the source code as parsed, add a specific flag to the declaration to indicate the fixity, as used in a Match now too.

Note: this flag is to capture the fixity used for the lexical definition of the type, primarily for use by ppr and ghc-exactprint.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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CC	mpickering
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