Fix is_member_function_pointer does not account for ellipsis. PR20345. Patch from Agustin Berge.

I reviewed the patch and added the test cases.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215984 91177308-0d34-0410-b5e6-96231b3b80d8
Add extra test case for PR20345. Should have been commited with r215984

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@215985 91177308-0d34-0410-b5e6-96231b3b80d8