Add workaround flag for lazy import-as-member
In rdar://123649082, a project failed to build because of the lazy import-as-member loading changes in #71320. That project was configured in a way that broke modularization and the correct solution is to fix it, but out of an abundance of caution, add a `-disable-named-lazy-import-as-member-loading` frontend flag in case a project needs to temporarily restore the old behavior.
As a bonus, this lets us write a test to verify that lazy import-as-member loading has positive performance impact.
