Usual disclaimer about how I'm being a bit of a bad kitty 😼 because "grab bag PRs" can be challenging to review and risky to merge. This PR is structured into fine-grained commits for easier reviewing and restricts itself to very low-risk changes (I have separate PRs for fixing libcxx failures).

• <optional>: Drop unused requires-expression parameters that appeared only in operator<=.
• <yvals.h>: _THROW should expand with parens.
  • Pre-existing since the beginning of time, noted by @CaseyCarter while reviewing Various cleanups: <chrono> #4119. clang-format produces throw(__VA_ARGS__) with no space, which is annoying but not worth suppressing.
• <condition_variable> and <thread> should immediately reject /clr:pure.
  • All other headers (e.g. <mutex>) follow this pattern.
• Comment that _Stoll, _Stoul, _Stoull are preserved for bincompat. Nobody declares or calls them, including via the CNAME macro.
  • Also, don't defend _Stoll's definition against macros - this is stl/src where we have complete control.
• <mdspan>: operator[] overloads can attach their open braces.
  • clang-format exhibits "sticky" behavior here - it's happy either way.
• <yvals_core.h>: Improve the _USE_EXTERN_CXX_EVERYWHERE_FOR_STL comment.
  • Suggested by @CaseyCarter while reviewing Add extern "C++" as a temporary workaround for #include/import coexistence #4154, applied without further changes.
• Drop low-value file comments and outdated "for ConcRT" comment.
  • This was the only remaining mention of ConcRT outside of ABI-preserved code. We don't need to preserve the "condition variable implementation" part for _Cnd_internal_imp_t as the type name is sufficiently clear.
  • This doesn't drop all file comments, just the ones that I thought were especially unnecessary (due to the filenames and file contents).
• Remove more unnecessary file comments.
  • Every single one of these was // MEOW function where MEOW appears below (without any macroization). Similar rationale to Remove SHOUTY BANNERS #2074 addressing STL: WE SHOULD REMOVE THE SHOUTY COMMENT BANNERS! #306, these comments are counterproductive in a world where editors like VSCode provide easy global search.
• Drop unnecessary/inconsistent newlines after open braces.
  • This doesn't attempt to drop all newlines after open braces - many of them are arguably providing clarity.
• Drop unnecessary/inconsistent newlines before #else.
  • Again, this doesn't attempt to drop all such newlines, many of which are providing clarity. The occurrences I'm changing, all involving _ITERATOR_DEBUG_LEVEL or _M_CEE_PURE, looked imbalanced and were ancient relics of codebase processing.