Thread Safety Analysis: major update to thread safety TIL.
Numerous changes, including:
  * Changed the way variables and instructions are handled in basic blocks to
    be more efficient.
  * Eliminated SExprRef.
  * Simplified futures.
  * Fixed documentation.
  * Compute dominator and post dominator trees.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217556 91177308-0d34-0410-b5e6-96231b3b80d8