[PM] Switch the CGSCC debug messages to use the standard LLVM debug

[PM] Switch the CGSCC debug messages to use the standard LLVM debug
printing techniques with a DEBUG_TYPE controlling them.

It was a mistake to start re-purposing the pass manager `DebugLogging`
variable for generic debug printing -- those logs are intended to be
very minimal and primarily used for testing. More detailed and
comprehensive logging doesn't make sense there (it would only make for
brittle tests).

Moreover, we kept forgetting to propagate the `DebugLogging` variable to
various places making it also ineffective and/or unavailable. Switching
to `DEBUG_TYPE` makes this a non-issue.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@310695 91177308-0d34-0410-b5e6-96231b3b80d8