Bug 1666525 - Update to clang 11.0.0 rc3 r=firefox-build-system-reviewers,rstewart

clang 11.0.0 rc3 has been tagged today and will likely be the final build.

Differential Revision: https://phabricator.services.mozilla.com/D91039

taskcluster/ci/fetch/toolchains.yml

@@ -425,11 +425,11 @@ clang-10:
         revision: d32170dbd5b0d54436537b6b75beaf44324e0c28
 
 clang-11:
-    description: clang 11.0.0 rc2 source code
+    description: clang 11.0.0 source code
     fetch:
         type: git
         repo: https://github.com/llvm/llvm-project
-        revision: 414f32a9e862b11f51063b75729278f8d81b12e9
+        revision: 8aca41f39c207b6f9efe2e448986d109892072ad
 
 civet-source:
     description: Repository for external static analysis checks