[clang] [llvm] [clang][hlsl] Add atan2 intrinsic part 1 (PR #107923)

[Farzon Lotfi via cfe-commits]

================
@@ -1244,6 +1244,12 @@ def ElementwiseATan : Builtin {
   let Prototype = "void(...)";
 }
 
+def ElementwiseATan2 : Builtin {
+  let Spellings = ["__builtin_elementwise_atan2"];
+  let Attributes = [NoThrow, Const, CustomTypeChecking];
----------------
farzonl wrote:

This is more a note for me:  We haven't been doing `CustomTypeChecking` for `"__builtin_hlsl_elementwise_*"` cases. Check to see if we should. I'm assuming `CustomTypeChecking` is used to have vector cases avoid impacting variadic argument cases.  

https://github.com/llvm/llvm-project/pull/107923