<yvals_core.h>: Use Standard _Pragma

[StephanTLavavej]

In <yvals_core.h>, the STL's central internal header, we define internal helper macros for temporarily suppressing [[deprecated]] warnings:

STL/stl/inc/yvals_core.h

Lines 448 to 468 in 811c8c1

	// clang-format off
	#ifndef _STL_DISABLE_DEPRECATED_WARNING
	#ifdef __clang__
	#define _STL_DISABLE_DEPRECATED_WARNING \
	_Pragma("clang diagnostic push") \
	_Pragma("clang diagnostic ignored \"-Wdeprecated-declarations\"")
	#else // __clang__
	#define _STL_DISABLE_DEPRECATED_WARNING \
	__pragma(warning(push)) \
	__pragma(warning(disable : 4996)) // was declared deprecated
	#endif // __clang__
	#endif // _STL_DISABLE_DEPRECATED_WARNING
	// clang-format on
	#ifndef _STL_RESTORE_DEPRECATED_WARNING
	#ifdef __clang__
	#define _STL_RESTORE_DEPRECATED_WARNING _Pragma("clang diagnostic pop")
	#else // __clang__
	#define _STL_RESTORE_DEPRECATED_WARNING __pragma(warning(pop))
	#endif // __clang__
	#endif // _STL_RESTORE_DEPRECATED_WARNING

For Clang, this uses Standard _Pragma to emit a #pragma directive within a macro. For MSVC, this uses non-Standard __pragma.

Our general rule is to avoid non-Standard extensions unless they're necessary. As of VS 2019 16.6 Preview 2, which the STL now requires, MSVC supports Standard _Pragma, so we should use it here. (We still need #ifdef __clang__ logic because Clang and MSVC need different syntax to suppress the [[deprecated]] warning.)

Note: Standard _Pragma takes a string while non-Standard __pragma doesn't, as the existing code demonstrates.

[StephanTLavavej]

We've discovered that this is blocked by a compiler bug - _Pragma doesn't work with /Zc:preprocessor (ironically). This is tracked by Microsoft-internal VSO-1091230 and VSO-1091758.

We'll need to wait for that compiler bug to be fixed, and then ship in a public Preview, before attempting to make this change again.