[clangd] Fix warning after fully covered enum switch. NFC.

By adding llvm_unreachable.

llvm-svn: 332233

clang-tools-extra/clangd/CodeComplete.cpp

@@ -828,6 +828,7 @@ bool allowIndex(CodeCompletionContext &CC) {
   case NestedNameSpecifier::Identifier:
     return false;
   }
+  llvm_unreachable("invalid NestedNameSpecifier kind");
 }
 
 } // namespace