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.