Change build_reduced_graph.rs so the fields def and module of NsDef are never both Some unless the NsDef represents a duplicate definition (see issue 26421).
This will break code that looks like:
struct Foo {
...
}
mod Foo {
...
}
Change this code to:
struct Foo {
...
}
impl Foo {
...
}
Or rename the module.
Closes#15205.
[breaking-change]
