[libc++] Add from-scratch testing configs for Windows

The paths to the compiler and to the python executable may need to
be quoted (if they're installed into e.g. C:\Program Files).

All testing commands that are executed expect a gcc compatible command
line interface, while clang-cl uses different command line options.
In the original testing config, if the chosen compiler was clang-cl, it
was replaced with clang++ by looking for such an executable in the path.

For the new from-scratch test configs, I instead chose to add
"--driver-mode=g++" to flags - invoking "clang-cl --driver-mode=g++"
has the same effect as invoking "clang++", without needing to run any
heuristics for picking a different compiler executable.

Differential Revision: https://reviews.llvm.org/D111202