Refactored driver so that any action that is implemented using an

ASTConsumer can also be verified using the diagnostics checker.  From
the command line, users may activate diagnostic checking using the
"-verify" option.  For example, "clang -verify -warn-dead-stores"
checks if the warnings flagged by the dead store checker match those
in the comments.

Note that we still have the option "-parse-ast-check" for backwards
comptability with existing test cases.  This option is now equivalent to 
"-parse-ast -verify".

llvm-svn: 42362