Break the cycle between Host and PluginProcessUtility.

There are only two users of NativeRegisterContextRegisterInfo,
and both are in process plugins.  Moving this code from Host
to Plugins/Process/Utility thus makes sense, and as it is the
only dependency from Host -> PluginProcessUtility, it also
breaks this cycle, reducing LLDB's overall cycle count from
45 to 44.

llvm-svn: 298466