Fix caching bug in MemoryBufferSerializedModuleLoader
By populating the memory cache before loading the module, we can avoid a cycle
where a module is imported that is an overlay, which then triggers
ClangImporter, which then (redundantly) triggers the import of the overlay
module, which would reimport the module again, since it's import is still
underway and it hasn't been entered into the cache yet.
rdar://118846313
