Local Windows builds to use clang-cl by default

[David Major]

Hi,

Local Windows builds will use clang-cl and lld-link by default as of
bug 1483835. This will make your builds match what has been shipping
on Nightly for the last several weeks, and help avoid the occasional
MSVC bustage that sneaks in now that those builds are tier-2.

If you already ran `mach bootstrap` for the recent cbindgen changes,
you'll already have an up-to-date clang and there's no further action
required.

If you need to revert to the Microsoft toolchain, you can put one or
both of these lines in your mozconfig (but please get in touch and
tell me why you needed to):
export CC=cl
export LINKER=link

Thanks!

[bsty...@gmail.com]