Bug 1512504 - Remove support for MSVC. r=froydnj

Consequently, this removes:
- MOZ_LIBPRIO, which is now always enabled.
- non_msvc_compiler, which is now always true.
- The cl.py wrapper, since it's not used anymore.
- CL_INCLUDES_PREFIX, which was only used for the cl.py wrapper.
- NONASCII, which was only there to ensure CL_INCLUDES_PREFIX still
  worked in non-ASCII cases.

This however keeps a large part of detecting and configuring for MSVC,
because we still do need it for at least headers, libraries, and midl.

Depends on D19614

Differential Revision: https://phabricator.services.mozilla.com/D19615

--HG--
extra : moz-landing-system : lando