Add support for Bazel builds on Windows with `clang-cl`.

Adds basic `--config=clang-cl` to set up the basic options needed, and
then fix a number of issues that surface in Windows builds for me.

With these fixes, `//llvm/...` builds cleanly. One unittest still fails,
but its just due to running out of stack space due to creating a large
number of short-lived stack variables. The test should probably be
decomposed into a set of tests (`LegalizerInfoTest::RuleSets`), but that
seemed like too invasive of a change here and with everything building
cleanly this isn't disrupting me experimenting with Windows builds.

Some parts of `//clang/...` builds, but that will require more work.