Resubmit "Improve Windows toolchain support for non-standard environments."

This resubmits change r220226.  That change broke the chromium
build bots because chromium it ships an hermetic MSVC toolchain
that it expects clang to fallback to by finding it on the path.

This patch fixes the issue by bumping up the prioritization of PATH
when looking for MSVC binaries.

Reviewed by: Hans Wennborg, Reid Kleckner
Differential Revision: http://reviews.llvm.org/D5892
Make a good guess about where MSVC and Windows SDK libraries are for linking.

When a user has not configured a standard Visual Studio environment
by running vcvarsall, clang tries its best to find Visual Studio
include files and executables anyway.  This patch makes clang also
try to find system and Windows SDK libraries for linking against,
as well.

Reviewed by: Hans Wennborg
Differential Revision: http://reviews.llvm.org/D5873