Thread safety analysis: Allow scoped releasing of capabilities

Summary:
The pattern is problematic with C++ exceptions, and not as widespread as
scoped locks, but it's still used by some, for example Chromium.

We are a bit stricter here at join points, patterns that are allowed for
scoped locks aren't allowed here. That could still be changed in the
future, but I'd argue we should only relax this if people ask for it.

Fixes PR36162.

Reviewers: aaron.ballman, delesley, pwnall

Reviewed By: delesley, pwnall

Subscribers: pwnall, cfe-commits

Differential Revision: https://reviews.llvm.org/D52578

llvm-svn: 349300