Revert "[C++20][Modules] Introduce an implementation module."

This reverts commit c6e9823724.

Reason: Broke the ASan buildbots, see https://reviews.llvm.org/D126959
(the original phabricator review) for more info.

clang/lib/CodeGen/CodeGenModule.cpp

@@ -548,8 +548,6 @@ void CodeGenModule::Release() {
     GlobalTopLevelStmtBlockInFlight = {nullptr, nullptr};
   }
 
-  // Module implementations are initialized the same way as a regular TU that
-  // imports one or more modules.
   if (CXX20ModuleInits && Primary && Primary->isInterfaceOrPartition())
     EmitCXXModuleInitFunc(Primary);
   else