[analyzer] Faster hashing of subsequences in CompoundStmts.

Summary: This patches improves the hashing subsequences in CompoundStmts by incrementally hashing all subsequences with the same starting position. This results in a reduction of the time for this constraint while running over SQLite from 1.10 seconds to 0.55 seconds (-50%).

Reviewers: NoQ

Reviewed By: NoQ

Subscribers: cfe-commits, xazax.hun, v.g.vassilev

Differential Revision: https://reviews.llvm.org/D34364

llvm-svn: 307509