Expose the ModuleFile in SerializedASTFile so that the underlying Module (and thus SILModule) can be retrieved when deserializing.

This is necessary since if one wishes to write a secondary tool with swift
headers, one can not access the ModuleFile in SerializedASTFile since to do so
would require you to be a friend of the class, something that would create a
build dependency in between the secondary tool and the swift libraries.

Swift SVN r14171

include/swift/Serialization/SerializedModuleLoader.h

@@ -142,6 +142,8 @@ class SerializedASTFile final : public LoadedFile {
     : LoadedFile(FileUnitKind::SerializedAST, M), File(file) {}
 
 public:
+  ModuleFile &getFile() { return File; }
+
   virtual void lookupValue(Module::AccessPathTy accessPath,
                            Identifier name, NLKind lookupKind,
                            SmallVectorImpl<ValueDecl*> &results) const override;