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

[via llvm-commits]

lntue wrote:

@jcranmer-intel  @hubert-reinterpretcast : The PR should be ready.  The issue was the incorrect `LLVM_READONLY` attribute in the `APFloat::exp` declaration.  I removed it in the latest commit and the new test passes now.

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