analyzer infrastructure: make a bunch of changes to symbolic expressions that

Zhongxing and I discussed by email.

Main changes:
- Removed SymIntConstraintVal and SymIntConstraint
- Added SymExpr as a parent class to SymbolData, SymSymExpr, SymIntExpr
- Added nonloc::SymExprVal to wrap SymExpr
- SymbolRef is now just a typedef of 'const SymbolData*'
- Bunch of minor code cleanups in how some methods were invoked (no functionality change)

This changes are part of a long-term plan to have full symbolic expression
trees. This will be useful for lazily evaluating complicated expressions.

llvm-svn: 67731