This PR avoids the need for a whole bunch of workarounds in constexpr <cmath> and should make it possible for /Zc:cmath TUs to coexist with /Zc:cmath- TUs.

• exp2 is completely ready to be _CONSTEXPR_CMATH26.
  • See LLVM libc's status at https://libc.llvm.org/headers/math/index.html .
• cmath: Mark float/long double overloads as _CONSTEXPR_CMATH26_NYI; the generic macros already are.
  • This will make it easier to remember to update these overloads in the future.
• cmath: constexpr inline => constexpr for lerp.
  • This is an unrelated cleanup while we're in the neighborhood, for an issue that was introduced on 2020-07-29 by Finish P0811R3 midpoint and lerp #1048.
• Implement the float versions of acosh, asinh, atanh, cosh, sinh, tanh, erf with LLVM libc, making them C++26 constexpr.
  • These are also ready (and we were using some builtins, just not making them constexpr).
• Instead of declaring MEOWl, define it as wrapping MEOW.
• Use extern "C" for decls, but extern "C++" with an inline namespace for defns.
  • This should avoid conflicts with the UCRT, while remaining invisible to users. This also avoids intrinsic headaches.
• Declare UCRT lgamma/tgamma instead of implementing with Boost.Math.
  • This avoids messing with the satellite DLL's export surface before we ship 14.52.
  • Resolves special_math.cpp: Investigate lgamma()/tgamma() error behavior #6417 by making it moot.