Unable to report an undeclared type behind a type-def at the proper location

[mhasel]

When parsing a type-def, we currently only store the name of the referenced type in the generated DataTypeDeclaration. If the aliased type does not exist, it is currently not possible to get back to the source-location of the definition, since we do not have a matching DataType in the index.
This leads to bad UX when validating undefined types, since the error messages shows on the user-type instead of where the actual error happened.
Additionally, ideally we would show such a validation for type-defs only once, instead of once per encountered user-type.

Here's an example:

TYPE myType : undeclaredType; END_TYPE
VAR_GLOBAL
    a: myType;
END_VAR

leads to

error[E052]: Unknown type: myType
      ┌─ demo.st:4:16
      │
    4 │             a: myType;
      │                ^^^^^^ Unknown type: myType