Export of internal Abseil changes.

--
b0015627610a311997ab21b8b295a8168b5915ec by Derek Mauro <dmauro@google.com>:

Add an empty log_severity library. The log_severity sources will
eventually migrate here once users are updated to depend on the new
target.

Note that no opensource user should be depending on log_severity.h
today since it has no purpose in opensource.

PiperOrigin-RevId: 258781702

--
c4d8d03f731d1de648ef2d17a55c3a16029ba331 by Derek Mauro <dmauro@google.com>:

Move atomic_hook.h to its own internal target.

PiperOrigin-RevId: 258765527
GitOrigin-RevId: b0015627610a311997ab21b8b295a8168b5915ec
Change-Id: I0ec9b8b3cb6a27dc32a5f6c318d07e02548f193d

absl/synchronization/BUILD.bazel

@@ -78,6 +78,7 @@ cc_library(
     deps = [
         ":graphcycles_internal",
         "//absl/base",
+        "//absl/base:atomic_hook",
         "//absl/base:base_internal",
         "//absl/base:config",
         "//absl/base:core_headers",

absl/synchronization/CMakeLists.txt

@@ -55,6 +55,7 @@ absl_cc_library(
     ${ABSL_DEFAULT_COPTS}
   DEPS
     absl::graphcycles_internal
+    absl::atomic_hook
     absl::base
     absl::base_internal
     absl::config