Thread Safety Analysis: add support for before/after annotations on mutexes.

Thread Safety Analysis: add support for before/after annotations on mutexes.
These checks detect potential deadlocks caused by inconsistent lock
ordering.  The checks are implemented under the -Wthread-safety-beta flag.

This patch also replaces calls to getAttrs() with calls to attrs() throughout
ThreadSafety.cpp, which fixes the earlier issue that cause assert failures.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@228051 91177308-0d34-0410-b5e6-96231b3b80d8