[clang] [llvm] [HLSL] Implement the `faceforward` intrinsic (PR #135878)

[Kaitlin Peng via cfe-commits]

================
@@ -105,35 +136,27 @@ bool SemaSPIRV::CheckSPIRVBuiltinFunctionCall(unsigned BuiltinID,
     if (SemaRef.checkArgCount(TheCall, 3))
       return true;
 
-    // check if the all arguments have floating representation
-    for (unsigned i = 0; i < TheCall->getNumArgs(); ++i) {
-      ExprResult Arg = TheCall->getArg(i);
-      QualType ArgTy = Arg.get()->getType();
-      if (!ArgTy->hasFloatingRepresentation()) {
-        SemaRef.Diag(Arg.get()->getBeginLoc(),
-                     diag::err_builtin_invalid_arg_type)
-            << i + 1 << /* scalar or vector */ 5 << /* no int */ 0 << /* fp */ 1
-            << ArgTy;
-        return true;
-      }
-    }
+    if (CheckAllArgsHaveFloatRepresentation(&SemaRef, TheCall))
+      return true;
 
-    // check if all arguments are of the same type
-    ExprResult A = TheCall->getArg(0);
-    ExprResult B = TheCall->getArg(1);
-    ExprResult C = TheCall->getArg(2);
-    if (!(SemaRef.getASTContext().hasSameUnqualifiedType(A.get()->getType(),
-                                                         B.get()->getType()) &&
-          SemaRef.getASTContext().hasSameUnqualifiedType(A.get()->getType(),
-                                                         C.get()->getType()))) {
-      SemaRef.Diag(TheCall->getBeginLoc(),
-                   diag::err_vec_builtin_incompatible_vector)
-          << TheCall->getDirectCallee() << /*useAllTerminology*/ true
-          << SourceRange(A.get()->getBeginLoc(), C.get()->getEndLoc());
+    if (CheckAllArgsHaveSameType(&SemaRef, TheCall))
----------------
kmpeng wrote:

You're right, it would be better to just check the first value then do `CheckAllArgsHaveSameType`.

The only caveat I can think of would be that it would change the error messages for tests, e.g. the error message for
```
float test_int_scalar_inputs2(float p0, int p1, int p2)
```
would change from `2nd argument must be a scalar or vector of floating-point types (was 'int')` to `all arguments to '__builtin_spirv_faceforward' must have the same type`, which I think is fine. 

If we want to make this change for `smoothstep` though, we would probably want to do it in a separate PR right?

https://github.com/llvm/llvm-project/pull/135878