[Analyzer][Core] Make SValBuilder to better simplify svals with 3 symbols in the tree

Add the capability to simplify more complex constraints where there are 3
symbols in the tree. In this change I extend simplifySVal to query constraints
of children sub-symbols in a symbol tree. (The constraint for the parent is
asked in getKnownValue.)

Differential Revision: https://reviews.llvm.org/D103317