[CFG] [analyzer] NFC: Allow more complicated construction contexts.

ConstructionContexts introduced in D42672 are an additional piece of information
included with CFGConstructor elements that help the client of the CFG (such as
the Static Analyzer) understand where the newly constructed object is stored.

The patch refactors the ConstructionContext class to prepare for including
multi-layered contexts that are being constructed gradually, layer-by-layer,
as the AST is traversed.

Differential Revision: https://reviews.llvm.org/D43428

llvm-svn: 325966