[Debugify] Add a mode to opt to enable faster testing

Opt's "-enable-debugify" mode adds an instance of Debugify at the
beginning of the pass pipeline, and an instance of CheckDebugify at the
end.

You can enable this mode with lit using: -Dopt="opt -enable-debugify".
Note that running test suites in this mode will result in many failures
due to strict FileCheck commands, etc.

It can be more useful to look for assertion failures which arise only
when Debugify is enabled, e.g to prove that we have (or do not have)
test coverage for some code path with debug info present.

Differential Revision: https://reviews.llvm.org/D41793

llvm-svn: 323256