Adjust msvc_stdlib_force_include.hpp to handle clang++

Summary: This patch adjusts the newly added `msvc_stdlib_force_include.hpp` so that it also works when used with `clang++`.

Reviewers: STL_MSFT

Reviewed By: STL_MSFT

Differential Revision: https://reviews.llvm.org/D28917

llvm-svn: 292539