Change CommandObjectPlatform commands to get the current platform
from the current Target, if there is one, else back off to getting
the currently selected platform from the Debugger (as it ws doing
previously.)

Remove code from DynamicLoaderDarwinKernel that was setting the platform
in both the Target and in the Debugger.


git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@178836 91177308-0d34-0410-b5e6-96231b3b80d8