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.

llvm-svn: 41779