Roll Clang from a93d03310e2c to 20d06c833d83 (flutter/engine#39152)

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/clang-flutter-engine
Please CC zra@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 Flutter: https://github.com/flutter/flutter/issues/new/choose

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

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

DEPS

@@ -40,7 +40,7 @@ vars = {
   # The list of revisions for these tools comes from Fuchsia, here:
   # https://fuchsia.googlesource.com/integration/+/HEAD/toolchain
   # If there are problems with the toolchain, contact fuchsia-toolchain@.
-  'clang_version': 'git_revision:a93d03310e2c02fa5e24544df4706650f85788f7',
+  'clang_version': 'git_revision:20d06c833d833ef6b2d0f519cc4a7998d49a2803',
 
   # When updating the Dart revision, ensure that all entries that are
   # dependencies of Dart are also updated to match the entries in the