[analyzer] Introduce IntSymExpr, where the integer is on the lhs.

Fix a bug in SimpleSValBuilder, where we should swap lhs and rhs when calling generateUnknownVal(), - the function which creates symbolic expressions when data is tainted. The issue is not visible when we only create the expressions for taint since all expressions are commutative from taint perspective.

Refactor SymExpr::symbol_iterator::expand() to use a switch instead of a chain of ifs.

llvm-svn: 146336