Roll Clang from 684052173971 to 6d12b954a7df

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/clang-dart-sdk
Please CC dart-engprod@google.com,dart-vm-gardener@grotations.appspotmail.com,dart-vm-team@google.com on the revert to ensure that a human
is aware of the problem.

To file a bug in Clang: https://bugs.fuchsia.dev/p/fuchsia/issues/list?q=component%3AToolchain
To file a bug in Dart SDK: https://github.com/dart-lang/sdk/issues

To report a problem with the AutoRoller itself, please file a bug:
https://issues.skia.org/issues/new?component=1389291&template=1850622

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/main/autoroll/README.md

Cq-Include-Trybots: luci.dart.try:dart-sdk-linux-try;luci.dart.try:dart-sdk-mac-try;luci.dart.try:dart-sdk-mac-arm64-try;luci.dart.try:dart-sdk-win-try
Change-Id: I3ba75dd58bda848c4262776824cada308f9bebab
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/405680
Commit-Queue: Alexander Aprelev <aam@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>
Reviewed-by: Alexander Aprelev <aam@google.com>

DEPS

@@ -76,7 +76,7 @@ vars = {
   "jsc_tag": "version:288804",
 
   # https://chrome-infra-packages.appspot.com/p/fuchsia/third_party/clang
-  "clang_version": "git_revision:684052173971868aab0e6b62d7770a6299e84141",
+  "clang_version": "git_revision:6d12b954a7df11e32acf110950d88bac282079b8",
 
   # https://chrome-infra-packages.appspot.com/p/gn/gn
   "gn_version": "git_revision:c97a86a72105f3328a540f5a5ab17d11989ab7dd",