[analyzer] Add double-unlock detection to PthreadLockChecker.

We've decided to punt on supporting recursive locks for now; the common case
is non-recursive.

Patch by Daniel Fahlgren!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205274 91177308-0d34-0410-b5e6-96231b3b80d8