[cxx-interop] Allow Darwin overlay to be rebuilt with C++ interop enabled

This removes a workaround from the module interface loader, which was forcing Darwin to be rebuilt from their textual interfaces with C++ interop disabled, even if the current compilation explicitly enables it.

See also 3791ccb.

rdar://142961112

Author: egorzhdan

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -2102,11 +2102,9 @@ InterfaceSubContextDelegateImpl::runInSubCompilerInstance(StringRef moduleName,
   BuildArgs.push_back("-module-name");
   BuildArgs.push_back(moduleName);
 
-  // FIXME: Hack for Darwin.swiftmodule, which cannot be rebuilt with C++
-  // interop enabled by the Swift CI because it uses an old host SDK.
   // FIXME: Hack for CoreGraphics.swiftmodule, which cannot be rebuilt because
   // of a CF_OPTIONS bug (rdar://142762174).
-  if (moduleName == "Darwin" || moduleName == "CoreGraphics") {
+  if (moduleName == "CoreGraphics") {
     subInvocation.getLangOptions().EnableCXXInterop = false;
     subInvocation.getLangOptions().cxxInteropCompatVersion = {};
     BuildArgs.erase(llvm::remove_if(BuildArgs,