[infra] Update to a clang with libc++.a for Mac ARM64.

Bug: #42773
Change-Id: I24e3e842ffc7b3ac28e3236b1b4ef393762f9c8a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/193823
Reviewed-by: Alexander Thomas <[email protected]>
Reviewed-by: William Hesse <[email protected]>
Commit-Queue: Alexander Thomas <[email protected]>

Author: rmacnak-google

DEPS

@@ -63,8 +63,8 @@ vars = {
   # The list of revisions for these tools comes from Fuchsia, here:
   # https://fuchsia.googlesource.com/integration/+/HEAD/prebuilts
   # If there are problems with the toolchain, contact fuchsia-toolchain@.
-  "clang_revision": "7e9747b50bcb1be28d4a3236571e8050835497a6",
-  "gn_revision": "1e3fd10c5df6b704fc764ee388149e4f32862823",
+  "clang_revision": "dc8d864e3a91938e11e8aea38bb43c9fdef361bc",
+  "gn_revision": "dba01723a441c358d843a575cb7720d54ddcdf92",
 
   # Scripts that make 'git cl format' work.
   "clang_format_scripts_rev": "c09c8deeac31f05bd801995c475e7c8070f9ecda",