[analyzer] Fix JSON dumps for location contexts.

Location context ID is a property of the location context, not of an item
within it. It's useful to know the id even when there are no items
in the context, eg. for the purposes of figuring out how did contents
of the Environment for the same location context changed across states.

Differential Revision: https://reviews.llvm.org/D62754

llvm-svn: 363895