[Sema] Add MacroQualified case for FunctionTypeUnwrapper

This is a fix for PR43315. An assertion error is hit for this minimal example:

```
//clang -cc1 -triple x86_64-- -S tstVMStructRC-min.cpp
int (a b)();  // Assertion `Chunk.Kind == DeclaratorChunk::Function' failed.
```

This is because we do not cover the case in the FunctionTypeUnwrapper where it
receives a MacroQualifiedType. We have not run into this earlier because this
is a unique case where the __attribute__ contains both __cdecl__ and
__regparm__ (in that order), and we are compiling for x86_64. Changing the
architecture or the order of __cdecl__ and __regparm__ does not raise the
assertion.

Differential Revision: https://reviews.llvm.org/D67992