Basic: rename VisualStudio to Windows

Visual Studio is the Integrated Development Environment.  The toolchain is
generally referred to MSVC.  Rename the target information to be more precise as
per the recommendation of Reid Kleckner.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@205609 91177308-0d34-0410-b5e6-96231b3b80d8