Revert "Improve Windows toolchain support for non-standard environments." (r220226)

In environments where PATH was set to point to the VS installation, Clang would
override that by looking in the registry and finding the latest VS installation.

If the environment is set up to point to a VS installation, that should take
precedence.

Reverting this until we can fix it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@220243 91177308-0d34-0410-b5e6-96231b3b80d8