Removed unused and broken template instantiation origin detection.

Having something like this would be useful in determining
whether to define a given template symbol during codegen, but
the frontend sometimes produces TemplateInstances in a scope
where sc->tinst has not been semantic'd, thus breaking the
chaining mechanism.
Do not codegen aggregate types from within debug info generation.

This avoids problems where we would codegen children of an
"inner" template instantiation (i.e. a member of a non-template
aggregate in another module) because we have no way to know the
outer (declare-only) entity exists in the respective
mustDefineSymbol invocation.

An example for this are the std.typecons.RefCounted internals of
std.file.DirIterator, as used from std.datetime and other modules.
This is not only inefficient, but also causes linking issues due
to attribute inference for these functions not having run yet
(and consequently the mangled name being different from the
actual definition).
Do not try to generate metadata for unresolved types.

This does not fix the root cause, which is likely a frontend
bug/oddity, but avoids the crash.