[analyzer] Provide .def-files and visitors for SVal/SymExpr/MemRegion.

Provide separate visitor templates for the three hierarchies, and also
the `FullSValVisitor' class, which is a union of all three visitors.

Additionally, add a particular example visitor, `SValExplainer', in order to
test the visitor templates. This visitor is capable of explaining the SVal,
SymExpr, or MemRegion in a natural language.

Differential Revision: http://reviews.llvm.org/D15448

llvm-svn: 257605