Update clang-cl driver for MSVC 2017.

2017 changes the way you find an installed copy of
Visual Studio as well as its internal directory layout.
As a result, clang-cl was unable to find VS2017 even
when you had run vcvarsall to set up a toolchain
environment.  This patch updates everything for 2017
and cleans up the way we handle a tiered search a la
environment -> installation -> PATH for which copy
of Visual Studio to bind to.

Patch originally by Hamza Sood, with some fixups for landing.

Differential Revision: https://reviews.llvm.org/D30758

llvm-svn: 297851