Thread safety analysis:

 * When we detect that a CFG block has inconsistent lock sets, point the
   diagnostic at the location where we found the inconsistency, and point a note
   at somewhere the inconsistently-locked mutex was locked.
 * Fix the wording of the normal (non-loop, non-end-of-function) case of this
   diagnostic to not suggest that the mutex is going out of scope.
 * Fix the diagnostic emission code to keep a warning and its note together when
   sorting the diagnostics into source location order.

llvm-svn: 149669