Front-end: add a new module loader that loads explicitly built Swift modules

To support -disable-implicit-swift-modules, the explicitly built modules
are passed down as compiler arguments. We need this new module loader to
handle these modules.

This patch also stops ModuleInterfaceLoader from building module from interface
when -disable-implicit-swift-modules is set.

lib/Serialization/SerializedModuleLoader.cpp

@@ -275,6 +275,12 @@ std::error_code SerializedModuleLoaderBase::openModuleDocFileIfPresent(
   return std::error_code();
 }
 
+std::unique_ptr<llvm::MemoryBuffer>
+SerializedModuleLoaderBase::getModuleName(ASTContext &Ctx, StringRef modulePath,
+                                          std::string &Name) {
+  return ModuleFile::getModuleName(Ctx, modulePath, Name);
+}
+
 std::error_code
 SerializedModuleLoaderBase::openModuleSourceInfoFileIfPresent(
     AccessPathElem ModuleID,