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

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,14 @@
+// RUN: %clang_cc1 -triple dxil-pc-shadermodel6.6-library  -x hlsl -finclude-default-header -verify %s
+
+
+int2 ToTwoInts(int V) {
+  return V.xy; // expected-error{{vector component access exceeds type 'int __attribute__((ext_vector_type(1)))' (vector of 1 'int' value)}}
+}
+
+float2 ToTwoFloats(float V) {
+  return V.rg; // expected-error{{vector component access exceeds type 'float __attribute__((ext_vector_type(1)))' (vector of 1 'float' value)}}
+}
+
+int4 SomeNonsense(int V) {
+  return V.poop; // expected-error{{illegal vector component name 'p'}}
----------------
AaronBallman wrote:

I'd like another test that validates we catch multiple levels of dots. e.g.,
```
float2 HowManyFloats(float V) {
  return V.rr.rr;
}
```
or dots not followed by anything useful:
```
float2 WhatIsHappening(float V) {
  return V.;
}
```

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