[analyzer] Unify SymbolVal and SymExprVal under a single SymbolVal

class.

We are going into the direction of handling SymbolData and other SymExpr
uniformly, so it makes less sense to keep two different SVal classes.
For example, the checkers would have to take an extra step to reason
about each type separately.

The classes have the same members, we were just using the SVal kind
field for easy differentiation in 3 switch statements. The switch
statements look more ugly now, but we can make the code more readable in
other ways, for example, moving some code into separate functions.

llvm-svn: 145833