[analyzer] Assert that nonloc::SymbolVal always wraps a non-Loc-type symbol.

In the current SVal hierarchy there are multiple ways of representing certain
values but few are actually used and expected to be seen by the code.

In particular, a value of a symbolic pointer is always represented by a
loc::MemRegionVal that wraps a SymbolicRegion that wraps the pointer symbol
and never by a nonloc::SymbolVal that wraps that symbol directly.

Assert the aforementioned fact. Fix one minor violation of it.

Differential Revision: https://reviews.llvm.org/D48205

llvm-svn: 337227