Use the new Windows environment for target detection

This follows the LLVM change to canonicalise the Windows target triple
spellings.  Rather than treating each Windows environment as a single entity,
the environments are now modelled properly as an environment.  This is a
mechanical change to convert the triple use to reflect that change.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@204978 91177308-0d34-0410-b5e6-96231b3b80d8