[clang] [clang] Make __builtin_exp and __builtin_expf constexpr. (PR #199808)

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lntue wrote:

> The implementation isn't meeting the [library.c]p3 restriction that exceptions (other than FE_INEXACT) cause library calls to not be constant expressions.

@jcranmer-intel I'm adding exception check for exp in APFloat at https://github.com/llvm/llvm-project/pull/203066. PTAL.  Thanks,

https://github.com/llvm/llvm-project/pull/199808