Consumed analysis: improve loop handling. The prior version of the analysis

Consumed analysis: improve loop handling.  The prior version of the analysis
marked all variables as "unknown" at the start of a loop.  The new version
keeps the initial state of variables unchanged, but issues a warning if the
state at the end of the loop is different from the state at the beginning.
This patch will eventually be replaced with a more precise analysis.

Initial patch by chris.wailes@gmail.com.  Reviewed and edited by
delesley@google.com.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192314 91177308-0d34-0410-b5e6-96231b3b80d8