[clang] [HLSL] Support vector swizzles on scalars (PR #67700)

[Chris B via cfe-commits]

================
@@ -0,0 +1,78 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.6-library  -x hlsl \
+// RUN:   -finclude-default-header -ast-dump %s | FileCheck %s
+
+
+// CHECK: ExtVectorElementExpr {{.*}} 'int __attribute__((ext_vector_type(2)))' xx
+// CHECK-NEXT: ImplicitCastExpr {{.*}} 'int __attribute__((ext_vector_type(1)))' lvalue <VectorSplat>
+// CHECK-NEXT: DeclRefExpr {{.*}} 'int' lvalue ParmVar {{.*}} 'V' 'int'
+
+int2 ToTwoInts(int V){
+  return V.xx;
+}
+
+// CHECK: ExtVectorElementExpr {{.*}} 'float __attribute__((ext_vector_type(4)))' rrrr
+// CHECK-NEXT: ImplicitCastExpr {{.*}} 'float __attribute__((ext_vector_type(1)))' lvalue <VectorSplat>
+// CHECK-NEXT: DeclRefExpr {{.*}} 'float' lvalue ParmVar {{.*}} 'V' 'float'
+
+
+float4 ToThreeFloats(float V){
+  return V.rrrr;
+}
+
+// CHECK: ExtVectorElementExpr {{.*}} 'int __attribute__((ext_vector_type(2)))' xx
+// CHECK-NEXT: ImplicitCastExpr {{.*}} 'int __attribute__((ext_vector_type(1)))' <VectorSplat>
+// CHECK-NEXT: IntegerLiteral {{.*}} 'int' 1
+
+int2 FillOne(){
+  return 1.xx;
+}
+
+
+// CHECK: ExtVectorElementExpr {{.*}} 'unsigned int __attribute__((ext_vector_type(3)))' xxx
+// CHECK-NEXT: ImplicitCastExpr {{.*}} 'unsigned int __attribute__((ext_vector_type(1)))' <VectorSplat>
+// CHECK-NEXT: IntegerLiteral {{.*}} 'unsigned int' 1
+
+uint3 FillOneUnsigned(){
+  return 1u.xxx;
+}
----------------
llvm-beanz wrote:

`HooBoy` fails because `wb` isn't a supported suffix in C++ mode (and all HLSL versions are C++-based). Also, even the most experimental HLSL version is only C++-11 based so the `auto` return type will always error. I've added a different integer test case:

```hlsl
int64_t4 HooBoy() {
  return 4l.xxxx;
}
```

`err()` works, which is also sadly expected despite the terrible syntax:
[FXC on shader playground](https://shader-playground.timjones.io/9111b2b533b5665e679c777fe4e5123b)


https://github.com/llvm/llvm-project/pull/67700