Frontend: add a front-end option to specify module names for which we prefer to loading via interfaces

ABI checker imports Swift frameworks by using Swift interfaces for various
reasons. The existing way of controlling preferred importing mechanism is by
setting an environment variable (SWIFT_FORCE_MODULE_LOADING), which may lead
to performance issues because the stdlib could also be loaded in this way.

This patch adds a new front-end option to specify module names for
which we prefer to importing via Swift interface. The option currently is only
accessible via swift-api-digester.

rdar://54559888

include/swift/Serialization/SerializedModuleLoader.h

@@ -42,8 +42,10 @@ class SerializedModuleLoaderBase : public ModuleLoader {
 protected:
   ASTContext &Ctx;
   ModuleLoadingMode LoadMode;
+  ArrayRef<std::string> PreferInterfaceForModules;
   SerializedModuleLoaderBase(ASTContext &ctx, DependencyTracker *tracker,
-                             ModuleLoadingMode LoadMode);
+                             ModuleLoadingMode LoadMode,
+                             ArrayRef<std::string> PreferInterfaceForModules = {});
 
   void collectVisibleTopLevelModuleNamesImpl(SmallVectorImpl<Identifier> &names,
                                              StringRef extension) const;