Enable WindowsToolChain on all native Windows builds

Make binaries built by MSVC, mingw and clang functionally equivalent. The
checks are trivially performed at runtime to eliminate functional differences
between supported configurations that used to be hard-coded.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@211461 91177308-0d34-0410-b5e6-96231b3b80d8