[Sema] Make FunctionType's TSI use unadjusted argument types

This helps preserve the type-as-written in the AST, which we need for
MSVC mangling.  In particular, we need to preserve the types of array
parameters in function pointer types.

The essence of this change is:
-  QualType ArgTy = Param->getType();
+  QualType ArgTy = Param->getTypeSourceInfo()->getType();

... followed by the adjustment in ActOnFunctionDeclarator().

Differential Revision: http://llvm-reviews.chandlerc.com/D883