[analyzer] Rename PruneNullReturnPaths to SuppressNullReturnPaths.

"Prune" is the term for eliminating pieces of a path that are not
relevant to the user. "Suppress" means don't show that path at all.
[analyzer] Add 'prune-paths' config option to disable path pruning.

This should be used for testing only. Path pruning is still on by default.
[analyzer] bugreporter::getDerefExpr now takes a Stmt, not an ExplodedNode.

This allows it to be used in places where the interesting statement
doesn't match up with the current node. No functionality change.
[analyzer] Track null object lvalues back through C++ method calls.

The expression 'a->b.c()' contains a call to the 'c' method of 'a->b'.
We emit an error if 'a' is NULL, but previously didn't actually track
the null value back through the 'a->b' expression, which caused us to
miss important false-positive-suppression cases, including
<rdar://problem/12676053>.