Bug 1625040 - Fix clang-5 toolchain task symbol. r=dmajor

Differential Revision: https://phabricator.services.mozilla.com/D68292

--HG--
extra : moz-landing-system : lando

taskcluster/ci/toolchain/clang.yml

@@ -13,7 +13,7 @@ job-defaults:
 linux64-clang-5.0:
     description: "Clang 5.0 toolchain build"
     treeherder:
-        symbol: TL(clang4.0)
+        symbol: TL(clang5.0)
     worker-type: b-linux-large
     run:
         script: build-clang.sh