[CMAKE] Override the flags for the Debug CMake configuration

Summary:
This diffs overrides the default flags for the Debug build type,
setting the optimization to -g3 -Og (or their fallbacks).

Depends on D2653

Test Plan:

mkdir buildcmake && cd buildcmake
cmake -GNinja .. -DCMAKE_BUILD_TYPE=Debug
ninja -v

Check that the -g3 -Og flags (or their fallbacks) flags are set in the
compile commands for the CXX compile lines. Check that the -g -O2
flags are set in the compile commands for the CC compile lines.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2644