CFG:
- Add 'LoopTarget' pointer field to CFGBlock. This records if the block is used
  as the 'loop back' path back to the head of a loop.
- For ForStmt, encode the loop back target as the increment code.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@70274 91177308-0d34-0410-b5e6-96231b3b80d8