Make WindowsToolChain portable

De-conditionalize as much as possible so we can start to fix this code.

No change in functionality.
WindowsToolChain: decouple build environment from the toolchain

Don't try to find the MSVC version that the binaries were built with. Doing so
defeats testing by causing invalid test passes on the build servers.

Whichever Visual Studio (or clang-cl.exe) edition was used to build the clang
package, it's strictly orthogonal and has no relation to software versions
available on the user's PC.