[analyzer] Aggressively cut back on the canonicalization in RegionStore.

Whenever we touch a single bindings cluster multiple times, we can delay
canonicalizing it until the final access. This has some interesting
implications, in particular that we shouldn't remove an /empty/ cluster
from the top-level map until canonicalization.

This is good for a 2% speedup or so on the test case in
<rdar://problem/12810842>

llvm-svn: 169523