[analyzer] First step toward removing

ConstraintManager::canReasonAbout() from the ExprEngine.

ExprEngine should not care if the constraint solver can reason about
something or not. The solver should be able to handle all the SymExprs.

To do this, the solver should be able to keep track of not only the
SymbolData but of all SymExprs. This is why we change SymbolRef to be an
alias of SymExpr*. When encountering an expression it cannot simplify,
the solver should just add the constraints to it.

llvm-svn: 145831