[Sema] Fix PR22637 - IndirectFieldDecl's discard qualifiers during template instantiation.

Summary:
Currently Clang fails to propagate qualifiers from the `CXXThisExpr` to the rebuilt `FieldDecl` for IndirectFieldDecls. For example:

```
template <class T> struct Foo {
  struct { int x; };
  int y;
  void foo() const { 
      static_assert(__is_same(int const&, decltype((y))));
      static_assert(__is_same(int const&, decltype((x)))); // assertion fails
  }
};
template struct Foo<int>;
```

The fix is to delegate rebuilding of the MemberExpr to `BuildFieldReferenceExpr` which correctly propagates the qualifiers.

Reviewers: rsmith, lebedev.ri, aaron.ballman, bkramer, rjmccall

Reviewed By: rjmccall

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D45412

llvm-svn: 329517