Static analyzer: Remove a bunch of outdated SymbolData objects and

Static analyzer: Remove a bunch of outdated SymbolData objects and
their associated APIs.  We no longer need separate SymbolData objects
for fields, variables, etc.  Instead, we now associated symbols with
the "rvalue" of a MemRegion (i.e., the value stored at that region).
Now we only have two kinds of SymbolData objects: SymbolRegionRValue
and SymbolConjured.

This cleanup also makes the distinction between a SymbolicRegion and a
symbolic value that is a location much clearer.  A SymbolicRegion
represents a chunk of symbolic memory, while a symbolic location is
just a "pointer" with different possible values.  Without any specific
knowledge, a symbolic location resolves (i.e., via a dereference) to a
SymbolicRegion.  In the future, when we do better alias reasoning, a
symbolic location can become an alias for another location, thus
merging the constraints on the referred SymbolicRegion with the other
region.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@62769 91177308-0d34-0410-b5e6-96231b3b80d8