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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@292539 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed