[clang][AST] Fix Wreturn-type gcc warning (NFC)

GCC warning:
```
/llvm-project/clang-tools-extra/clangd/SemanticHighlighting.cpp: In function ‘bool clang::clangd::{anonymous}::canHighlightName(clang::DeclarationName)’:
/llvm-project/clang-tools-extra/clangd/SemanticHighlighting.cpp:64:1: warning: control reaches end of non-void function [-Wreturn-type]
   64 | }
      | ^
```

Author: nullptr-cpp

clang-tools-extra/clangd/SemanticHighlighting.cpp

@@ -61,6 +61,7 @@ bool canHighlightName(DeclarationName Name) {
   case DeclarationName::CXXUsingDirective:
     return false;
   }
+  llvm_unreachable("invalid name kind");
 }
 
 llvm::Optional<HighlightingKind> kindForType(const Type *TP);