[Strict memory safety] Improve handling of safe types nested within unsafe ones

[DougGregor]

Fix a number of issues related to strict memory safety's handling of safe types nested within unsafe ones, including:

• Don't propagate @unsafe from a type to its nested types automatically
• A reference to a safe type nested within an unsafe type is safe; don't diagnose it as unsafe
• Typealiases cannot be meaningfully unsafe if their underlying type isn't
• Lots of standard library updates that remove extraneous unsafe uses

This clears up a number of false positives with strict memory safety that I've encountered in practice, tracked by rdar://149628760

[DougGregor]

@swift-ci please smoke test