[analyzer] Simplify function SVal::getAsSymbolicExpression and similar ones

Summary: Simplify functions SVal::getAsSymbolicExpression SVal::getAsSymExpr and SVal::getAsSymbol. After revision I concluded that `getAsSymbolicExpression` and `getAsSymExpr` repeat functionality of `getAsSymbol`, thus them can be removed.

Fix: Remove functions SVal::getAsSymbolicExpression and SVal::getAsSymExpr.

Differential Revision: https://reviews.llvm.org/D85034