Created by: ghvg1313
Adds some common settings that makes sense for Debug config, including:
- 'DEAD_CODE_STRIPPING': 'NO'
- 'ONLY_ACTIVE_ARCH': 'YES'
- 'GCC_SYMBOLS_PRIVATE_EXTERN': 'NO'
Also fix the issue where default flags are wiped out if user defined custom project configs in the target level.
With the old approach (user defined config override the default configs), Buck have no context which config is the debug version, so now default configs are merged with the user-defined ones, with debug having the extra flags above.