[test] support machinery changes for EDG & C1XX /Za

This change works around a couple of bugs:

1. EDG doesn't like explicit constexpr in a derived class. This program:

  struct Base {};
  struct Derived : Base {
      constexpr Derived() = default;
  };

  triggers "error: defaulted default constructor cannot be constexpr."

2. C1XX with /Za has no idea which constructor needs to be valid for copy elision.

The change also conditionally disables parts of the msvc_stdlib_force_include.hpp header that conflict with external configuration when _LIBCXX_IN_DEVCRT is defined.

Differential Revision: https://reviews.llvm.org/D32778

llvm-svn: 302707