[clang] [llvm] [HLSL] Implement the `smoothstep` intrinsic (PR #132288)

[Kaitlin Peng via llvm-commits]

================
@@ -101,6 +101,52 @@ bool SemaSPIRV::CheckSPIRVBuiltinFunctionCall(unsigned BuiltinID,
     TheCall->setType(RetTy);
     break;
   }
+  case SPIRV::BI__builtin_spirv_smoothstep: {
+    if (SemaRef.checkArgCount(TheCall, 3))
+      return true;
+
+    // check if the all arguments have floating representation
+    ExprResult A = TheCall->getArg(0);
+    QualType ArgTyA = A.get()->getType();
+    if (!ArgTyA->hasFloatingRepresentation()) {
+      SemaRef.Diag(A.get()->getBeginLoc(),
+                   diag::err_typecheck_convert_incompatible)
----------------
kmpeng wrote:

Don't we also want to say the [api supports matrices](https://github.com/llvm/llvm-project/pull/132288#discussion_r2008116154) though? I don't think I can do that with `err_builtin_invalid_arg_type` unless I edit `DiagnosticSemaKinds.td`—the closest I can get is this message:
```
1st argument must be a scalar or vector of floating-point types (was 'int')
```

https://github.com/llvm/llvm-project/pull/132288