[clang] Emit constrained atan2 intrinsic for clang builtin (PR #113636)

[Farzon Lotfi via cfe-commits]

farzonl wrote:

So the minute this lands our change is going to be visible across all backends.

Should we add any constraint intrinsic tests to `PowerPC`, `RISCV`, `SystemZ`, etc?

Here are just a handful of tests that we might want to update, but there are others:

- llvm/test/CodeGen/PowerPC/ctrloop-constrained-fp.ll
- llvm/test/CodeGen/RISCV/double-intrinsics-strict.ll l
- lvm/test/CodeGen/SystemZ/vector-constrained-fp-intrinsics.ll

https://github.com/llvm/llvm-project/pull/113636