[CFG] Add LoopExit information to CFG

This patch introduces a new CFG element CFGLoopExit that indicate when a loop
ends. It does not deal with returnStmts yet (left it as a TODO).
It hidden behind a new analyzer-config flag called cfg-loopexit (false by
default).
Test cases added.

The main purpose of this patch right know is to make loop unrolling and loop
widening easier and more efficient. However, this information can be useful for
future improvements in the StaticAnalyzer core too.

Differential Revision: https://reviews.llvm.org/D35668

llvm-svn: 311235