[analyzer] Fix constraint being dropped when analyzing a program without taint tracking enabled

Summary:
This patch removes the constraint dropping when taint tracking is disabled.

It also voids the crash reported in D28953 by treating a SymSymExpr with non pointer symbols as an opaque expression.

Updated the regressions and verifying the big projects now; I'll update here when they're done.

Based on the discussion on the mailing list and the patches by @ddcc.

Reviewers: george.karpenkov, NoQ, ddcc, baloghadamsoftware

Reviewed By: george.karpenkov

Subscribers: delcypher, llvm-commits, rnkovacs, xazax.hun, szepet, a.sidorin, ddcc

Differential Revision: https://reviews.llvm.org/D48650

llvm-svn: 337167