[mlir][tblgen] Fix crash in `-gen-enum-(decls|defs)` when `EnumAttrInfo` is not defined

[zero9178]

Summary

This should instead simply produce an empty file instead of crashing and causing a build to fail. This allows one to be more lazy in cmake code and just always generate the files for enum attributes, even if none are ever defined in the dialect.

[jpienaar]

Thanks!

mlir/test/mlir-tblgen/enums-gen-empty.td

+// RUN: mlir-tblgen -gen-enum-decls -I %S/../../include %s
+// RUN: mlir-tblgen -gen-enum-defs -I %S/../../include %s

mlir/tools/mlir-tblgen/EnumsGen.cpp

)";
 
   // Emit DenseMapInfo for this enum class
   emitDenseMapInfo(enumName, underlyingType, cppNamespace, os);
 }
 
 static bool emitEnumDecls(const RecordKeeper &recordKeeper, raw_ostream &os) {
   llvm::emitSourceFileHeader("Enum Utility Declarations", os);
 
-  auto defs = recordKeeper.getAllDerivedDefinitions("EnumAttrInfo");
+  auto defs = recordKeeper.getAllDerivedDefinitionsIfDefined("EnumAttrInfo");
   for (const auto *def : defs)
     emitEnumDecl(*def, os);
 
   return false;
 }
 
 static void emitEnumDef(const Record &enumDef, raw_ostream &os) {
   EnumAttr enumAttr(enumDef);
static void emitEnumDef(const Record &enumDef, raw_ostream &os) {
   for (auto ns : llvm::reverse(namespaces))
     os << "} // namespace " << ns << "\n";
   os << "\n";
 }
 
 static bool emitEnumDefs(const RecordKeeper &recordKeeper, raw_ostream &os) {
   llvm::emitSourceFileHeader("Enum Utility Definitions", os);
 
-  auto defs = recordKeeper.getAllDerivedDefinitions("EnumAttrInfo");
+  auto defs = recordKeeper.getAllDerivedDefinitionsIfDefined("EnumAttrInfo");
   for (const auto *def : defs)
     emitEnumDef(*def, os);
 
   return false;
 }
 
 // Registers the enum utility generator to mlir-tblgen.
 static mlir::GenRegistration