Stop leaking memory from Module and FileUnit.

Also, disallow creating Modules and FileUnits on the stack. They must always
live as long as the ASTContext.

<rdar://problem/15596964>

Swift SVN r13671

include/swift/Serialization/SerializedModuleLoader.h

@@ -132,6 +132,8 @@ class SerializedASTFile final : public LoadedFile {
 
   ModuleFile &File;
 
+  ~SerializedASTFile() = default;
+
   SerializedASTFile(Module &M, ModuleFile &file)
     : LoadedFile(FileUnitKind::SerializedAST, M), File(file) {}