Thread-safety analysis: adding in a basic lockset tracking system. This

Thread-safety analysis: adding in a basic lockset tracking system. This
system flags an error when unlocking a lock which was not held, locking
the same lock twice, having a different lockset on each iteration of a
loop, or going out of scope while still holding a lock. In order to
successfully use the lockset, this patch also makes sure that attribute
arguments are attached correctly for later parsing.

This patch was also worked on by DeLesley Hutchins.

Note: This patch has been reviewed by Chandler Carruth and Jeffrey
Yasskin. Feel free to provide post-commit review comments for a
subsequent patch.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@138350 91177308-0d34-0410-b5e6-96231b3b80d8