Position of instances/types are the same as the position of the def (and they should be the same as the AST node)

Our general framework is:
AST node -> instance/type -> def
E.g.,
X10CanonicalTypeNode_c -> X10ParsedClassType_c/ConstrainedType_c -> X10ClassDef_c
The position info of instance/type should be those of the AST node,
but they are copied from the def in:
asType() and asInstance()
(e.g., ClassDef.asType())

We should either refactor so instance/type won't have position info,
or fix this bug (by creating instance from an AST node)