[clang] [Clang] Add constexpr eval for cmath builtins (PR #194327)

[Zahira Ammarguellat via cfe-commits]

https://github.com/zahiraam commented:

Adding this here as github didnt' let me add it at line #20227 of `ExprConstant.cpp`.
What does the `frexp` helper return for special values?  Also a check to ensure the mantissa is valid is missing.

https://github.com/llvm/llvm-project/pull/194327