Implement is_constant_evaluated(), more feature-test macros, and LWG issues

[StephanTLavavej]

Description

• Fix P0595R2 is_constant_evaluated() #156: WG21-P0595 is_constant_evaluated()
  • This is enabled for Clang 9 and VS 2019 16.5 Preview 2 (the first preview with _MSC_VER set to 1925) because it depends on __builtin_is_constant_evaluated. Thanks to Jennifer Yao who implemented this in C1XX (the MSVC compiler front-end).
  • Currently, we need additional preprocessor logic to disable this for the Microsoft-internal "checked-in toolset"; that logic looks strange because the checked-in toolset's version is currently 19.25.28318.97 while a freshly-built compiler is (for example) 19.25.26504.99.
• Fix P1902R1 Missing Feature-Test Macros 2017-2019 #339: WG21-P1902 Missing Feature-Test Macros 2017-2019
  • For unimplemented features affected by this paper, I've updated all of their GitHub issues.
  • For implemented features affected by this paper, I'm adding feature-test macros here. All of those features were directly controlled by _HAS_CXX20 and are gaining new macros (instead of having existing macros increased).
  • Note that __cpp_lib_ssize is slightly special, as that paper also patched span. I believe that we should simply define the feature-test macro now, given its name.
• Fix LWG-3199 istream >> bitset<0> fails #70: LWG-3199 istream >> bitset<0> fails
• Fix LWG-3257 Missing feature testing macro update from WG21-P0858 #335: LWG-3257 Missing feature testing macro update from WG21-P0858
• Fix LWG-3266 to_chars(bool) should be deleted #332: LWG-3266 to_chars(bool) should be deleted
• Reorder and rewrap yvals_core.h comments.

Checklist

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• Identifiers in product code changes are properly _Ugly as per
  https://eel.is/c++draft/lex.name#3.1 or there are no product code changes.
• The STL builds successfully and all tests have passed (must be manually
  verified by an STL maintainer before automated testing is enabled on GitHub,
  leave this unchecked for initial submission).
• These changes introduce no known ABI breaks (adding members, renaming
  members, adding virtual functions, changing whether a type is an aggregate
  or trivially copyable, etc.).
• These changes were written from scratch using only this repository,
  the C++ Working Draft (including any cited standards), other WG21 papers
  (excluding reference implementations outside of proposed standard wording),
  and LWG issues as reference material. If they were derived from a project
  that's already listed in NOTICE.txt, that's fine, but please mention it.
  If they were derived from any other project (including Boost and libc++,
  which are not yet listed in NOTICE.txt), you must mention it here,
  so we can determine whether the license is compatible and what else needs
  to be done.