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

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@344096 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed