[SemaCXX] Param diagnostic matches overload logic

Summary:
Given the following test program:

```
class C {
public:
  int A(int a, int& b);
};

int C::A(const int a, int b) {
  return a * b;
}
```

Clang would produce an error message that correctly diagnosed the
redeclaration of `C::A` to not match the original declaration (the
parameters to the two declarations do not match -- the original takes an
`int &` as its 2nd parameter, but the redeclaration takes an `int`). However,
it also produced a note diagnostic that inaccurately pointed to the
first parameter, claiming that `const int` in the redeclaration did not
match the unqualified `int` in the original. The diagnostic is
misleading because it has nothing to do with why the program does not
compile.

The logic for checking for a function overload, in
`Sema::FunctionParamTypesAreEqual`, discards cv-qualifiers before
checking whether the types are equal. Do the same when producing the
overload diagnostic.

Reviewers: rsmith

Reviewed By: rsmith

Subscribers: cpplearner, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57032

llvm-svn: 352831