Annotate scoped_lock as with scoped_lockable attribute

Summary:
Scoped capabilities need to be annotated as such, otherwise the thread
safety analysis won't work as intended.

Fixes PR39234.

Reviewers: ldionne

Reviewed By: ldionne

Subscribers: christof, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53049

llvm-svn: 344096