This implements the protocols and static functions proposed in SE-0246, plus some initial test coverage. It also has some rough accompanying cleanup of tgmath. It does not include the globals (on scalars or SIMD types) nor does it deprecate much in tgmath.h.
* Revert "Merge pull request #23791 from compnerd/you-know-nothing-clang"
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* Revert "Merge pull request #23780 from compnerd/math-is-terrible"
This reverts commit 2d7fedd25f, reversing
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* Revert "Merge pull request #23140 from stephentyrone/mafs"
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This commit implements SE-0246, by adding conformance to Real to the Float, CGFloat, Double, and Float80 types, implemented either in terms of the system's C math library, existing standard library functionality, or LLVM intrinsics. It includes basic test coverage for these new functions, and deprecates and obsoletes *some* existing functionality in the Platform overlay. We still need to make a decision about how to handle the remaining "tgmath" functions, because obsoleting them is technically a source-breaking change (if users have unqualified names like "exp(1)", it's fine, but it would break users who have used qualified names like "Darwin.exp(1)".)
