[Driver] Use UniversalCRT on Windows if available

Summary:
With Visual Studio 2015 release, a part of runtime library was extracted
and now comes with Windows Kits. This patch enables clang to use
Universal CRT library if  %INCLUDE or %LIB environment varaibles are not
specified.

See also https://llvm.org/bugs/show_bug.cgi?id=24741

Patch by Igor Kudrin

Reviewers: zturner, hans, rnk

Subscribers: ruiu, cfe-commits

Differential Revision: http://reviews.llvm.org/D12695

llvm-svn: 247362