Implement support for equality comparisons (!=, ==) of member

Implement support for equality comparisons (!=, ==) of member
pointers, by extending the "composite pointer type" logic to include
member pointer types.

Introduce test cases for member pointer comparisons, including those
that involve the builtin operator candidates implemented earlier.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@79925 91177308-0d34-0410-b5e6-96231b3b80d8