`<xfacet>`: Consider remove `<yvals.h>` inclusion

[frederick-vs-ja]

<xfacet> is built into the import library. However, currently it includes <yvals.h> and thus not a core header.

It seems that <xfacet> only needs to obtain these macro definitions from <yvals.h>:

STL/stl/inc/yvals.h

Lines 315 to 331 in b5df16a

	#ifndef _CRTIMP2_PURE
	#ifdef _M_CEE_PURE
	#define _CRTIMP2_PURE
	#else
	#define _CRTIMP2_PURE _CRTIMP2
	#endif
	#endif // !defined(_CRTIMP2_PURE)
	#ifndef _CRTIMP2_IMPORT
	#if defined(CRTDLL2) && defined(_CRTBLD)
	#define _CRTIMP2_IMPORT __declspec(dllexport)
	#elif defined(_DLL) && !defined(_STATIC_CPPLIB)
	#define _CRTIMP2_IMPORT __declspec(dllimport)
	#else // ^^^ defined(_DLL) && !defined(_STATIC_CPPLIB) / !defined(_DLL) || defined(_STATIC_CPPLIB) vvv
	#define _CRTIMP2_IMPORT
	#endif // ^^^ !defined(_DLL) || defined(_STATIC_CPPLIB) ^^^
	#endif // !defined(_CRTIMP2_IMPORT)

So I guess it's feasible to make <xfacet> a core header.

Likewise, awint.hpp only needs _CRTIMP2 from <crtdefs.h>, so it might also be possible to make it core (see also #5444). However, I'm not sure whether <crtdefs.h> is suitable for core-ness.

[StephanTLavavej]

The thing is that _CRTIMP2_PURE is one of the main macros for accessing separately compiled code. The original intent of "core header" was "those without any runtime dependencies" (like <type_traits>) although I think we've blurred the lines a bit by also using it to enforce "safe for injection into the import lib".

I am inclined to not change how this works. It's better to make it very hard to access separately compiled code from core headers, at the cost of preserving the exceptions for <xfacet> (which is old and very weird/hazardous to health) and <awint.hpp>.

[AlexGuteniev]

The original intent of "core header" was "those without any runtime dependencies" (like <type_traits>)

Was #4991 in a wrong direction? Normally it will have inline memcpy, but in debug and under some weird non-debug option it will link to memcpy function

[StephanTLavavej]

Now that you point it out, I'm not super comfortable with it, correct.

"Without any runtime dependencies" is the stricter notion, so anything beyond that is pushing it.