[clang] [llvm] Add emitAtomicCompareExchangeBuiltin helper function (PR #101966)

[Nikita Popov via cfe-commits]

nikic wrote:

The TLI dependency in Clang CodeGen is a pretty substantial departure from how things currently work -- with the major implication that you can no longer reliably run codegen for a target for which the corresponding LLVM backend has not been compiled. Currently, codegen tests assume that this is not required and that they can be run without `REQUIRES: *-registered-target`.

https://github.com/llvm/llvm-project/pull/101966