[clang] [clang-tools-extra] [Attributes][HLSL] Teach EnumArgument to refer to an external enum (PR #70835)

[Justin Bogner via cfe-commits]

================
@@ -1010,14 +1022,17 @@ namespace {
       // trivial because some enumeration values have multiple named
       // enumerators, such as type_visibility(internal) and
       // type_visibility(hidden) both mapping to TypeVisibilityAttr::Hidden.
-      OS << "const char *" << getAttrName() << "Attr::Convert" << type
-         << "ToStr(" << type << " Val) {\n"
+      OS << "const char *" << getAttrName() << "Attr::Convert" << shortType
+         << "ToStr(" << fullType << " Val) {\n"
          << "  switch(Val) {\n";
       SmallDenseSet<StringRef, 8> Uniques;
       for (size_t I = 0; I < enums.size(); ++I) {
         if (Uniques.insert(enums[I]).second)
-          OS << "  case " << getAttrName() << "Attr::" << enums[I]
-             << ": return \"" << values[I] << "\";\n";
+          OS << "  case " << fullType << "::" << enums[I] << ": return \""
+             << values[I] << "\";\n";
+      }
+      if (isExternal) {
+        OS << "  default: llvm_unreachable(\"Invalid attribute value\");\n";
----------------
bogner wrote:

In `Attrs.inc` the only change is that we don't emit a declaration of the enum. In `AttrImpl.inc` we get something like this:
```c++
const char *HLSLResourceAttr::ConvertToStr(llvm::hlsl::ResourceClass Val) {
  switch(Val) {
  case llvm::hlsl::ResourceClass::SRV: return "SRV";
  case llvm::hlsl::ResourceClass::UAV: return "UAV";
  case llvm::hlsl::ResourceClass::CBuffer: return "CBuffer";
  case llvm::hlsl::ResourceClass::Sampler: return "Sampler";
  default: llvm_unreachable("Invalid attribute value");
  }
  llvm_unreachable("No enumerator with that value");
}
```

Where previously the enum values were spelled like `HLSLResourceAttr::ResourceClass::SRV` and the `default` label wasn't present.


https://github.com/llvm/llvm-project/pull/70835