[Driver] Enable --rtlib option for MSVC target

This enables "--rtlib compiler-rt" option under MSVC environment.

Patch by Roman Shirokiy.

Differential Revision: http://reviews.llvm.org/D17453


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@263422 91177308-0d34-0410-b5e6-96231b3b80d8