The PGO stage in bug 1326486 made the win64-clang-cl toolchain take even longer than it already did. And unfortunately, this impacts the lead time of even the cross-compiled builds, because linux64-clang-9-win-cross depends on win64-clang-cl for its compiler-rt runtimes and a Windows-flavored clang-cl.exe for MIDL under Wine. In code review glandium suggested that the win-cross toolchain could settle for the runtimes and clang-cl.exe from a single-stage build. That would allow the cross-compile toolchain to start more quickly.

Currently the linux64-clang-9-win-cross toolchain depends on the win64-clang-cl toolchain for a few files. This causes very long lead times when toolchains are rebuilt, because of the un-parallelizable chain of tasks win64-clang-cl -> linux64-clang-9-win-cross -> builds.

As a partial mitigation, this patch adds a single-stage clang-cl build for consumption by the cross toolchain. It's not a very high quality build, but good enough for the purpose it serves, while being faster to build.

https://hg.mozilla.org/mozilla-central/rev/e49ad3cdea17