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.

include/swift/Frontend/ModuleInterfaceLoader.h

@@ -127,6 +127,38 @@ class LangOptions;
 class SearchPathOptions;
 class CompilerInvocation;
 
+/// A ModuleLoader that loads explicitly built Swift modules specified via
+/// -swift-module-file
+class ExplicitSwiftModuleLoader: public SerializedModuleLoaderBase {
+  explicit ExplicitSwiftModuleLoader(ASTContext &ctx, DependencyTracker *tracker,
+                                     ModuleLoadingMode loadMode,
+                                     bool IgnoreSwiftSourceInfoFile);
+  std::error_code findModuleFilesInDirectory(
+    AccessPathElem ModuleID,
+    const SerializedModuleBaseName &BaseName,
+    SmallVectorImpl<char> *ModuleInterfacePath,
+    std::unique_ptr<llvm::MemoryBuffer> *ModuleBuffer,
+    std::unique_ptr<llvm::MemoryBuffer> *ModuleDocBuffer,
+    std::unique_ptr<llvm::MemoryBuffer> *ModuleSourceInfoBuffer) override;
+
+  bool isCached(StringRef DepPath) override { return false; };
+
+  struct Implementation;
+  Implementation &Impl;
+public:
+  static std::unique_ptr<ExplicitSwiftModuleLoader>
+  create(ASTContext &ctx,
+         DependencyTracker *tracker, ModuleLoadingMode loadMode,
+         ArrayRef<std::string> ExplicitModulePaths,
+         bool IgnoreSwiftSourceInfoFile);
+
+  /// Append visible module names to \p names. Note that names are possibly
+  /// duplicated, and not guaranteed to be ordered in any way.
+  void collectVisibleTopLevelModuleNames(
+      SmallVectorImpl<Identifier> &names) const override;
+  ~ExplicitSwiftModuleLoader();
+};
+
 struct ModuleInterfaceLoaderOptions {
   bool remarkOnRebuildFromInterface = false;
   bool disableInterfaceLock = false;
@@ -137,6 +169,7 @@ struct ModuleInterfaceLoaderOptions {
     disableImplicitSwiftModule(Opts.DisableImplicitModules) {}
   ModuleInterfaceLoaderOptions() = default;
 };
+
 /// A ModuleLoader that runs a subordinate \c CompilerInvocation and
 /// \c CompilerInstance to convert .swiftinterface files to .swiftmodule
 /// files on the fly, caching the resulting .swiftmodules in the module cache