[analyzer] NFC: Merge code for finding and tracking construction target.

When analyzing C++ code, a common operation in the analyzer is to discover
target region for object construction by looking at CFG metadata ("construction
contexts"), and then track the region path-sensitively until object construction
is resolved, where the amount of information, again, depends on construction
context.

Scan construction context only once for both purposes.

Differential Revision: https://reviews.llvm.org/D47304

llvm-svn: 334678