[analyzer] Add numerous assertions to SVal, SymExpr, and MemRegion classes.

Clean up vtable anchors (remove anchors for regions that have regular
out-of-line virtual methods, add anchors for regions that don't have those).

Fix private/public methods (all constructors should now be private for leaf
classes, protected for abstract classes).

No functional change intended, only extra sanity checks and cleanups.

Differential Revision: https://reviews.llvm.org/D26837

llvm-svn: 300187