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).
This not only reduces code duplication, but the unification
also enables code a la StructLiteralExp to handle classes
(for CTFE class constant support in 2.063).
