[lldb][test] TestDataFormatterCpp.py: add test-case for member function pointer format

This patch adds a test for formatting of member function pointers.
This was split from https://reviews.llvm.org/D145242, which caused
this test case to fail on Windows buildbots.

I split this out in order to make sure that this indeed works on Windows
without the D145242 patch.

Differential Revision: https://reviews.llvm.org/D145487