Test "core" headers

[StephanTLavavej]

See: https://github.com/microsoft/STL/wiki/The-Difference-Between-Core-And-Non-Core-Headers

During the development of #1370, I damaged the "core" header <type_traits> by including <cstdint> (which we found just before merging). I didn't even check <cstdint> because it was "obviously safe" - it just includes <stdint.h> and has using-declarations for typedefs. However, it was also including <yvals.h> (having predated the "core"/"non-core" split), which contains the STL's #pragma detect_mismatch directives:

STL/stl/inc/cstdint

Line 9 in 5f736ef

#include <yvals.h>

We should have a test that includes all of the headers we expect to be "core", and verifies that they are.