[CFG] [analyzer] Add construction contexts for loop condition variables.

Loop condition variables, eg.

  while (shared_ptr<int> P = getIntPtr()) { ... })

weren't handled in r324794 because they don't go through the common
CFGBuilder::VisitDeclStmt method. Which means that they regressed
after r324800.

Fix the regression by duplicating the necessary construction context scan in
the loop visiting code.

Differential Revision: https://reviews.llvm.org/D45706

llvm-svn: 330382