[ms-cxxabi] Properly mangle member pointers

There were three things missing from the original implementation:

- We would omit the 'E' qualifier for members int 64-bit mode.
- We would not exmaine the qualifiers in 'IsMember' mode.
- We didn't generate the correct backref to the base class.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@187753 91177308-0d34-0410-b5e6-96231b3b80d8