CodeGen: rename Win64 ExceptionHandling to WinEH

This exception format is not specific to Windows x64.  A similar approach is
taken on nearly all architectures.  Generalise the name to reflect reality.
This will eventually be used for Windows on ARM data emission as well.

Switch the enum and namespace into an enum class.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@212000 91177308-0d34-0410-b5e6-96231b3b80d8