PR14759: Debug info support for C++ member pointers.

This works fine with GDB for member variable pointers, but GDB's support for
member function pointers seems to be quite unrelated to
DW_TAG_ptr_to_member_type. (see GDB bug 14998 for details)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171698 91177308-0d34-0410-b5e6-96231b3b80d8