Refactored driver options that perform analyses/work over CFGs to use

Refactored driver options that perform analyses/work over CFGs to use
"CFGVisitor", which now handles all the boilerplate for iterating over
the function definitions in a translation unit and building the CFGs.
This logic was previously replicated for each driver option that used
CFGs.

The options -dump-cfg, -view-cfg, -check-dead-stores, and
-dump-live-variables now use this refactored code.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@41779 91177308-0d34-0410-b5e6-96231b3b80d8