PR14759: Improve/correct support for debug info for C++ member pointers.

Using added LLVM functionality in r171698. This works in GDB for member
variable pointers but not member function pointers. See the LLVM commit and
GDB bug 14998 for details.

Un-xfailing cases in the GDB 7.5 test suite will follow.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@171699 91177308-0d34-0410-b5e6-96231b3b80d8