Bug 1493812 - Update clang version used for the grcov Mac build. r=me

Pushing on a CLOSED TREE because this is fixing the grcov Mac toolchain task.

--HG--
extra : histedit_source : be97081d5eec36dea7d25680c2590282ebf8dab2

taskcluster/ci/toolchain/macosx.yml

@@ -151,4 +151,4 @@ macosx64-grcov:
         toolchain-artifact: public/build/grcov.tar.xz
     toolchains:
         - linux64-rust-1.28
-        - linux64-clang-6
+        - linux64-clang-7