[Windows] Simplify WindowsSupport.h

Sink Windows version detection code from WindowsSupport.h to Path.inc.
These functions don't need to be inlined. I randomly picked Process.inc
for the Windows version helpers, since that's the most related file.

Sink MakeErrMsg to Program.inc since it's the main client.

Move those functions into the llvm namespace, and delete the scoped
handle copy and assignment operators.

Reviewers: zturner, aganea

Differential Revision: https://reviews.llvm.org/D54182

llvm-svn: 346280