AST: Add GenericSignature field to FunctionTypeRepr
AST: Add GenericSignature field to FunctionTypeRepr

This will be used to wean the SILParser off contextual types.

It is unfortunate that we now bloat each FunctionTypeRepr by
16 bytes to allow generic function types to be written down
in SIL. Perhaps we could use a different TypeRepr subclass
for function types in SIL mode.

khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift,khizkhiz/swift