[CFG] [analyzer] NFC: Enumerate construction context layer kinds.

This is a refactoring patch; no functional change intended.

The common part of ConstructionContextLayer and ConstructedObjectKey is
factored out into a new structure, ConstructionContextItem.

Various sub-kinds of ConstructionContextItem are enumerated in order to
provide richer information about construction contexts.

Differential Revision: https://reviews.llvm.org/D49210.

llvm-svn: 338439