Allow struct field default values to reference struct's generics
Right now, the default field value feature (https://github.com/rust-lang/rust/issues/132162) lowers anon consts whose types may reference ADT params that the const doesn't inherit.
This PR fixes this, so that these defaults can reference ADTs' generics, and sets the `generics_of` parenting up correctly.
There doesn't seem to be a good reason not to support this, since the anon const has a well-defined type from the field, and the anon const doesn't interact with the type system like generic parameter defaults do.
r? `````@boxyuwu````` or reassign
I could also make this into an error if this seems problematic (https://github.com/rust-lang/rust/compare/master...compiler-errors:rust:default-field-value-implicit-param?expand=1)...... but I'd rather make this work and register an open question on the tracking issue about validating that this is well-vetted.
Fixes#137896