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/ModuleFile.cpp

@@ -2190,6 +2190,30 @@ TypeDecl *ModuleFile::lookupLocalType(StringRef MangledName) {
   return cast<TypeDecl>(getDecl(*iter));
 }
 
+std::unique_ptr<llvm::MemoryBuffer>
+ModuleFile::getModuleName(ASTContext &Ctx, StringRef modulePath,
+                          std::string &Name) {
+  // Open the module file
+  auto &fs = *Ctx.SourceMgr.getFileSystem();
+  auto moduleBuf = fs.getBufferForFile(modulePath);
+  if (!moduleBuf)
+    return nullptr;
+
+  // Load the module file without validation.
+  std::unique_ptr<ModuleFile> loadedModuleFile;
+  ExtendedValidationInfo ExtInfo;
+  bool isFramework = false;
+  serialization::ValidationInfo loadInfo =
+     ModuleFile::load(modulePath.str(),
+                      std::move(moduleBuf.get()),
+                      nullptr,
+                      nullptr,
+                      /*isFramework*/isFramework, loadedModuleFile,
+                      &ExtInfo);
+  Name = loadedModuleFile->Name;
+  return std::move(loadedModuleFile->ModuleInputBuffer);
+}
+
 OpaqueTypeDecl *ModuleFile::lookupOpaqueResultType(StringRef MangledName) {
   PrettyStackTraceModuleFile stackEntry(*this);