[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@183614 91177308-0d34-0410-b5e6-96231b3b80d8
[ms-cxxabi] Use desugared TSI in the mangler instead of canonical types

At this point, it's clear that the MSVC mangler uses the type-as-written
instead of the canonical type, so this should bring us closer to MSVC.

In particular, this makes the existing array parameter code simpler and
more correct.

Fixes PR16096.

This also fixes a separate issue that Richard spotted in review.
Because separate declarations of the same entity can be spelled and
mangled differently, MSVC always mangles the earliest declaration in an
attempt to avoid link errors.  Clang now does the same.

Differential Revision: http://llvm-reviews.chandlerc.com/D844