[lit] Support running tests on Windows without GnuWin32

Historically, we have told contributors that GnuWin32 is a pre-requisite
because our tests depend on utilities such as sed, grep, diff, and more.
However, Git on Windows includes versions of these utilities in its
installation.  Furthermore, GnuWin32 has not been updated in many years.
For these reasons, it makes sense to have the ability to run llvm tests
in a way that is both:
  a) Easier on the user (less stuff to install)
  b) More up-to-date (The verions that ship with git are at least as
     new, if not newer, than the versions in GnuWin32.

We add support for this here by attempting to detect where Git is
installed using the Windows registry, confirming the existence of
several common Unix tools, and then adding this location to lit's PATH
environment.

Differential Revision: https://reviews.llvm.org/D84380