[Sema] Allow int arguments for attributes to be enum values (#6353)

The SPIR-V backend uses `ValidateAttributeIntArg` for the
`vk::ext_builtin_input` and `vk::ext_builtin_output` attrs, as well as
some others. It is convenient for users to be able to pass in an enum
value to this attribute, for example:

```
enum class BuiltIn {
  HelperInvocation = 23
};

[[vk::ext_builtin_input(BuiltIn::HelperInvocation)]]
static const bool gl_HelperInvocation;
```

Fixes #6337

tools/clang/lib/Sema/SemaHLSL.cpp

@@ -12334,7 +12334,7 @@ static int ValidateAttributeIntArg(Sema &S, const AttributeList &Attr,
     } else {
       if (ArgNum.isInt()) {
         value = ArgNum.getInt().getSExtValue();
-        if (!(E->getType()->isIntegralType(S.Context)) || value < 0) {
+        if (!(E->getType()->isIntegralOrEnumerationType()) || value < 0) {
           S.Diag(Attr.getLoc(), diag::warn_hlsl_attribute_expects_uint_literal)
               << Attr.getName();
         }

tools/clang/test/CodeGenSPIRV/spv.inline.builtin.enum.hlsl

@@ -0,0 +1,18 @@
+// RUN: %dxc -T cs_6_0 -E main -fcgl  %s -spirv -verify
+
+// expected-no-diagnostics
+
+enum class BuiltIn {
+  HelperInvocation = 23
+};
+
+[[vk::ext_builtin_input(BuiltIn::HelperInvocation)]]
+static const bool gl_HelperInvocation;
+
+uint square_x(uint3 v) {
+  return v.x * v.x;
+}
+
+[numthreads(32,1,1)]
+void main() {
+}