Dwarf: preserve deduped struct navs

[jacobly0]

fn Foo(comptime T: type) type {
    return struct {
        x: T,
        const Bar = struct { x: u32 };
    };
}
pub fn main() void {
    var foo1: Foo(u32) = .{ .x = 1 };
    var foo2: Foo(bool) = .{ .x = false };
    _ = .{ &foo1, &foo2, @TypeOf(foo1).Bar, @TypeOf(foo2).Bar };
    @breakpoint();
}

(lldb) p @TypeOf(foo1)
(type) struct {}
(lldb) p @TypeOf(foo2)
(type) struct {
  Bar = struct {}
}

↓

(lldb) p @TypeOf(foo1)
(type) struct {
  Bar = struct {}
}
(lldb) p @TypeOf(foo2)
(type) struct {
  Bar = repro.repro.Foo(u32).Bar
}

Also, start porting stage2 pretty printers to self-hosted debug info and thread-safe internpool changes.