[analyzer] Handle SymbolCast in SValBuilder

Make the SimpleSValBuilder to be able to look up and use a constraint
for an operand of a SymbolCast, when the operand is constrained to a
const value.
This part of the SValBuilder is responsible for constant folding. We
need this constant folding, so the engine can work with less symbols,
this way it can be more efficient. Whenever a symbol is constrained with
a constant then we substitute the symbol with the corresponding integer.
If a symbol is constrained with a range, then the symbol is kept and we
fall-back to use the range based constraint manager, which is not that
efficient. This patch is the natural extension of the existing constant
folding machinery with the support of SymbolCast symbols.

Differential Revision: https://reviews.llvm.org/D126481