Bug 1834955 - Upgrade minimum supported Clang-cl version to 9....

Bug 1834955 - Upgrade minimum supported Clang-cl version to 9. r=firefox-build-system-reviewers,sergesanspaille

We have less constraints on the minimum supported clang-cl version, and
it would unlock C++17 inline variables (which had a bug on Windows
targets in clang up-to 8.x).

Differential Revision: https://phabricator.services.mozilla.com/D179037