Bug 1650239 - Build llvm-dsymutil out of the clang-10 source tree r=firefox-build-system-reviewers,rstewart

The current version hits OOM errors when dsymutil-ing files created by clang 10 and 11.

The choice of clang-10 here is somewhat arbitrary in that it's a fetch job that we conveniently already had in the tree. It doesn't have to be exactly version 10 specifically.

Differential Revision: https://phabricator.services.mozilla.com/D82453

taskcluster/ci/fetch/toolchains.yml

@@ -317,13 +317,6 @@ hfsplus-tools:
         sha256: b01b203a97f9a3bf36a027c13ddfc59292730552e62722d690d33bd5c24f5497
         size: 411205
 
-llvm-for-dsymutil:
-    description: llvm source code for dsymutil
-    fetch:
-        type: git
-        repo: https://github.com/llvm/llvm-project
-        revision: 3b7811f6441be13c9f613f81ef93297d231b4f8e
-
 rust-size:
     description: rust-size source code
     fetch:

taskcluster/ci/toolchain/misc.yml

@@ -33,7 +33,7 @@ linux64-llvm-dsymutil:
         toolchain-artifact: public/build/llvm-dsymutil.tar.xz
     fetches:
         fetch:
-            - llvm-for-dsymutil
+            - clang-10
         toolchain:
             - linux64-gcc-7