Thread safety analysis: Allow relockable scopes

Summary:
It's already allowed to prematurely release a scoped lock, now we also
allow relocking it again, possibly even in another mode.

This is the second attempt, the first had been merged as r339456 and
reverted in r339558 because it caused a crash.

Reviewers: delesley, aaron.ballman

Reviewed By: delesley, aaron.ballman

Subscribers: hokein, cfe-commits

Differential Revision: https://reviews.llvm.org/D49885

llvm-svn: 340459