* Clean up debug preprocessor macros
This change removes the `DBG` macro value in favor of `NDEBUG`, and
also converts many uses of `_DEBUG` with `NDEBUG`.
_DEBUG is set by MSVC when debug information generation is enabled.
NDEBUG is set by CMake debug configurations and by LLVM on all
configurations when `LLVM_ENABLE_ASSERTIONS` is set.
Aligning with LLVM's NDEBUG usage allows a consistent pattern
throughout the codebase.
* Address PR feedback
fb873d0a14