ADT/Triple: Helper to determine if we are targeting the Windows CRT

Summary:
This support will be utilized in things like clang to help check printf
format specifiers that are only valid when using the VSCRT.

Reviewers: rnk, asl, chandlerc

Reviewed By: chandlerc

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1455

llvm-svn: 188935