Change ObjCContainerDecl to contain the entire range for the '@end'

piece of the declaration.  The '@' and the 'end' are separate tokens,
and require two SourceLocations to accurately track.

This change was motivated because ObjCContainerDecl::getSourceRange()
would previously not return the entire range of the declaration (the
'end' would be left off).

llvm-svn: 92891