Run test in a freestanding environment so it doesn't accidentally pick up system headers for the wrong target.

While there add a test that verifies that the header parses in C++ mode.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@164679 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed