[clang][codegen] Mention the invariant that LLVM demangler should be able to handle mangled names generated by clang.

https://discourse.llvm.org/t/rfc-clang-diagnostic-for-demangling-failures/82835/8

Author: VitaNuo

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2047,6 +2047,13 @@ StringRef CodeGenModule::getMangledName(GlobalDecl GD) {
                  GD.getWithKernelReferenceKind(KernelReferenceKind::Kernel),
                  ND));
 
+  // This invariant should hold true in the future.
+  // Prior work: https://discourse.llvm.org/t/rfc-clang-diagnostic-for-demangling-failures/82835/8
+  // https://github.com/llvm/llvm-project/issues/111345
+  // assert(llvm::isMangledName(MangledName) &&
+  //        llvm::demangle(MangledName) != MangledName &&
+  //        "LLVM demangler must demangle clang-generated names");
+
   auto Result = Manglings.insert(std::make_pair(MangledName, GD));
   return MangledDeclNames[CanonicalGD] = Result.first->first();
 }