[clang] [clang] constexpr `__builtin_elementwise_abs` support (PR #152497)

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Mr-Anyone wrote:

@RKSimon For the floating-point part, does this simply call `llvm::abs` on each element, or is there a specific standard it needs to follow?

https://github.com/llvm/llvm-project/blob/ccd35e5b2f1438646d11c27eaa475f34681f9cbe/llvm/include/llvm/ADT/APFloat.h#L1557-L1561

https://github.com/llvm/llvm-project/pull/152497