[TableGen] Fix __CLAUSE_NO_CLASS macro leak in directive emitter (llvm#82912)

`__CLAUSE_NO_CLASS` was not undefined inside the
`GEN_CLANG_CLAUSE_CLASS` block, resulting in macro redifinition warnings
when several generated directives are used simultaneously.

Author: FruitClover

llvm/test/TableGen/directive1.td

@@ -258,6 +258,7 @@ def TDL_DirA : Directive<"dira"> {
 // IMPL-EMPTY:
 // IMPL-NEXT:  #undef __IMPLICIT_CLAUSE_NO_CLASS
 // IMPL-NEXT:  #undef __IMPLICIT_CLAUSE_CLASS
+// IMPL-NEXT:  #undef __CLAUSE_NO_CLASS
 // IMPL-NEXT:  #undef __CLAUSE
 // IMPL-NEXT:  #undef CLAUSE_NO_CLASS
 // IMPL-NEXT:  #undef CLAUSE_CLASS

llvm/test/TableGen/directive2.td

@@ -206,6 +206,7 @@ def TDL_DirA : Directive<"dira"> {
 // IMPL-EMPTY:
 // IMPL-NEXT:  #undef __IMPLICIT_CLAUSE_NO_CLASS
 // IMPL-NEXT:  #undef __IMPLICIT_CLAUSE_CLASS
+// IMPL-NEXT:  #undef __CLAUSE_NO_CLASS
 // IMPL-NEXT:  #undef __CLAUSE
 // IMPL-NEXT:  #undef CLAUSE_NO_CLASS
 // IMPL-NEXT:  #undef CLAUSE_CLASS

llvm/utils/TableGen/DirectiveEmitter.cpp

@@ -842,6 +842,7 @@ static void GenerateClauseClassMacro(const DirectiveLanguage &DirLang,
   OS << "\n";
   OS << "#undef __IMPLICIT_CLAUSE_NO_CLASS\n";
   OS << "#undef __IMPLICIT_CLAUSE_CLASS\n";
+  OS << "#undef __CLAUSE_NO_CLASS\n";
   OS << "#undef __CLAUSE\n";
   OS << "#undef CLAUSE_NO_CLASS\n";
   OS << "#undef CLAUSE_CLASS\n";