[NFC][TableGen] Add InclusionGuardEmitter to emit header inclusion guards

Add a RAII  class `InclusionGuardEmitter` which is similar to
`IfDefEmitter` but emits header inclusion guards, and adopt it it
DirectiveEmitter.

Author: jurahul

llvm/include/llvm/TableGen/CodeGenHelpers.h

@@ -34,6 +34,22 @@ class IfDefEmitter {
   raw_ostream &OS;
 };
 
+// Simple RAII helper for emitting header inclusion guard (i.e,
+// ifndef-define-endif).
+class InclusionGuardEmitter {
+public:
+  InclusionGuardEmitter(raw_ostream &OS, StringRef Name)
+      : Name(Name.str()), OS(OS) {
+    OS << "#ifndef " << Name << "\n"
+       << "#define " << Name << "\n\n";
+  }
+  ~InclusionGuardEmitter() { OS << "\n#endif // " << Name << "\n\n"; }
+
+private:
+  std::string Name;
+  raw_ostream &OS;
+};
+
 // Simple RAII helper for emitting namespace scope. Name can be a single
 // namespace (empty for anonymous namespace) or nested namespace.
 class NamespaceEmitter {

llvm/test/TableGen/directive1.td

@@ -177,6 +177,7 @@ def TDL_DirA : Directive<[Spelling<"dira">]> {
 // CHECK-NEXT:    static constexpr bool is_iterable = true;
 // CHECK-NEXT:  };
 // CHECK-NEXT:  } // namespace llvm
+// CHECK-EMPTY:
 // CHECK-NEXT:  #endif // LLVM_Tdl_INC
 
 

llvm/test/TableGen/directive2.td

@@ -150,6 +150,7 @@ def TDL_DirA : Directive<[Spelling<"dira">]> {
 // CHECK-NEXT:    static constexpr bool is_iterable = true;
 // CHECK-NEXT:  };
 // CHECK-NEXT:  } // namespace llvm
+// CHECK-EMPTY:
 // CHECK-NEXT:  #endif // LLVM_Tdl_INC
 
 // IMPL:      #ifdef GEN_FLANG_DIRECTIVE_CLAUSE_SETS

llvm/utils/TableGen/Basic/DirectiveEmitter.cpp

@@ -266,10 +266,9 @@ static void emitDirectivesDecl(const RecordKeeper &Records, raw_ostream &OS) {
     return;
 
   StringRef Lang = DirLang.getName();
+  InclusionGuardEmitter IncGuard(OS, (Twine("LLVM_") + Lang + "_INC").str());
 
-  OS << "#ifndef LLVM_" << Lang << "_INC\n";
-  OS << "#define LLVM_" << Lang << "_INC\n";
-  OS << "\n#include \"llvm/ADT/ArrayRef.h\"\n";
+  OS << "#include \"llvm/ADT/ArrayRef.h\"\n";
 
   if (DirLang.hasEnableBitmaskEnumInNamespace())
     OS << "#include \"llvm/ADT/BitmaskEnum.h\"\n";
@@ -370,7 +369,6 @@ static void emitDirectivesDecl(const RecordKeeper &Records, raw_ostream &OS) {
     OS << "};\n";
   }
   LlvmNS.close();
-  OS << "#endif // LLVM_" << Lang << "_INC\n";
 }
 
 // Given a list of spellings (for a given clause/directive), order them