DFATAL log level is not implemented

[mumbleskates]

Describe the bug

LOG(DFATAL) is incompletely implemented. ::absl::kLogDebugFatal is referenced in the macro definition of ABSL_LOGGING_INTERNAL_LOG_DFATAL, and DFATAL log level is mentioned several other places, but kLogDebugFatal is not defined anywhere and there seem to be other pieces of the puzzle missing.

Note that LOG(DFATAL) is not the same as DLOG(FATAL): the latter is compiled out in the presence of NDEBUG, while the former (which this bug report is about) is demoted to be equivalent to LOG(ERROR).

Steps to reproduce the bug

#include "absl/log/log.h"
int main() {
  LOG(DFATAL) << "compilation failure";
}

misc/scratch.cc:3:3: error: expected ';' after expression
  LOG(DFATAL) << "compilation failure";
  ^
external/com_google_absl/absl/log/log.h:168:7: note: expanded from macro 'LOG'
      ABSL_LOGGING_INTERNAL_LOG_##severity.InternalStream()
      ^
<scratch space>:401:1: note: expanded from here
ABSL_LOGGING_INTERNAL_LOG_DFATAL
^
external/com_google_absl/absl/log/internal/strip.h:59:3: note: expanded from macro 'ABSL_LOGGING_INTERNAL_LOG_DFATAL'
  ::absl::log_internal::LogMessage(__FILE__, __LINE__, ::absl::kLogDebugFatal)
  ^
misc/scratch.cc:3:3: error: use of undeclared identifier 'STATELESS'
external/com_google_absl/absl/log/log.h:167:42: note: expanded from macro 'LOG'
  ABSL_LOG_INTERNAL_CONDITION_##severity(STATELESS, true) \
                                         ^
misc/scratch.cc:3:3: error: no member named 'kLogDebugFatal' in namespace 'absl'
  LOG(DFATAL) << "compilation failure";
  ^~~~~~~~~~~
external/com_google_absl/absl/log/log.h:168:7: note: expanded from macro 'LOG'
      ABSL_LOGGING_INTERNAL_LOG_##severity.InternalStream()
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
<scratch space>:401:1: note: expanded from here
ABSL_LOGGING_INTERNAL_LOG_DFATAL
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
external/com_google_absl/absl/log/internal/strip.h:59:64: note: expanded from macro 'ABSL_LOGGING_INTERNAL_LOG_DFATAL'
  ::absl::log_internal::LogMessage(__FILE__, __LINE__, ::absl::kLogDebugFatal)
                                                       ~~~~~~~~^
3 errors generated.

What version of Abseil are you using?

master: 9398fa76a3436bfb89f7a6ec5c74a21f1e4d6029

What operating system and version are you using

ubuntu 22.04.1

What compiler and version are you using?

Ubuntu clang version 16.0.0-++20220921031234+9f13b9346b7c-1~exp1~20220921151331.436 Target: x86_64-pc-linux-gnu Thread model: posix InstalledDir: /usr/bin Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/10 Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11 Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/9 Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/11 Candidate multilib: .;@m64 Selected multilib: .;@m64

What build system are you using?

Build label: 5.3.1 Build target: bazel-out/k8-opt/bin/src/main/java/com/google/devtools/build/lib/bazel/BazelServer_deploy.jar Build time: Mon Sep 19 17:28:49 2022 (1663608529) Build timestamp: 1663608529 Build timestamp as int: 1663608529

Additional context

This is a potential blocking regression for a migration away from glog.