Validation of the input side of FunctionTypeRepr was previously being done in Sema because of expression folding. If we instead push the invariant that the input TypeRepr should always be a TupleTypeRepr into the AST a number of nice cleanups fall out:
- The SIL Parser no longer accepts Swift 2-style type declarations
- Parse is more cleanly able to reject invalid FunctionTypeReprs
- Clients of the AST can be assured the input type is always a TupleType so we can flush Swift 2 hacks
