logging: rename the `DEBUG` log level to `DBG`
Summary:
I ran into some open source projects that define `DEBUG` as a preprocessor
macro on the compiler command line. (They effectively defined it as the
opposite of `NDEBUG`.) Doing some Google searches revealed that there are a
number of projects that appear to use this as a macro.
Therefore this diff renames the `DEBUG` log level enum value to `DBG` to avoid
potentially conflicting with projects that do use `DEBUG` as a macro name.
I did keep the behavior that `logLevelToString()` returns "DEBUG" for an input
of `LogLevel::DBG`.
Reviewed By: yfeldblum
Differential Revision:
D6690465
fbshipit-source-id:
35bb1698afb45eb670e60c192f21390cbf09331d