[clang] [Clang] Add __builtin_bswapg (PR #162433)

[Oliver Hunt via cfe-commits]

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ojhunt wrote:

Haha, I just saw the file list and thought "that's an odd file for you to add" but I see constant-builtins-2.c already exists \o/ :D

That aside, please add C++ tests :D

The C++ tests should include explicit template parameters, auto typed parameters (in a lambda or similar), and (while I suspect this should be trivially correct given the existing correct const evaluation tests) tests for builtin_swapg as value typed template parameters.

https://github.com/llvm/llvm-project/pull/162433