[cxx-interop] Support CxxStdlib overlay for libc++ on Linux

This teaches Swift to rebuild the CxxStdlib overlay module from its interface when using a C++ standard library that is not the platform default, specifically libc++ on Linux.

rdar://138838506

lib/Frontend/ModuleInterfaceLoader.cpp

@@ -2035,6 +2035,15 @@ InterfaceSubContextDelegateImpl::InterfaceSubContextDelegateImpl(
 
     GenericArgs.push_back(
         ArgSaver.save("-cxx-interoperability-mode=" + compatVersion));
+
+    if (!langOpts.isUsingPlatformDefaultCXXStdlib() &&
+        langOpts.CXXStdlib == CXXStdlibKind::Libcxx) {
+      genericSubInvocation.getLangOptions().CXXStdlib = CXXStdlibKind::Libcxx;
+      genericSubInvocation.getClangImporterOptions().ExtraArgs.push_back(
+          "-stdlib=libc++");
+      GenericArgs.push_back("-Xcc");
+      GenericArgs.push_back("-stdlib=libc++");
+    }
   }
 }