This adds toolchain definitions for clang 11.0.0 rc2, so that developers can get a sneak peek, but nothing in automation uses these tasks yet. We'll make the switch in a later patch.
NB: most of `clang.yml` is rote copy-paste, except for `macosx64-clang-11` which makes a deliberate departure, described in a comment.
Differential Revision: https://phabricator.services.mozilla.com/D88189
David Major