Detect struct construction with private field in field with default

When trying to construct a struct that has a public field of a private type, suggest using `..` if that field has a default value.

```
error[E0603]: struct `Priv1` is private
  --> $DIR/non-exhaustive-ctor.rs:25:39
   |
LL |     let _ = S { field: (), field1: m::Priv1 {} };
   |                            ------     ^^^^^ private struct
   |                            |
   |                            while setting this field
   |
note: the struct `Priv1` is defined here
  --> $DIR/non-exhaustive-ctor.rs:14:4
   |
LL |    struct Priv1 {}
   |    ^^^^^^^^^^^^
help: the field `field1` you're trying to set has a default value, you can use `..` to use it
   |
LL |     let _ = S { field: (), .. };
   |                            ~~
```

1 files changed, 1 insertions, 0 deletions

diff --git a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
index df95ed602cd..11fef3be5d0 100644
--- a/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
+++ b/compiler/rustc_metadata/src/rmeta/decoder/cstore_impl.rs
@@ -395,6 +395,7 @@ provide! { tcx, def_id, other, cdata,
 
     crate_extern_paths => { cdata.source().paths().cloned().collect() }
     expn_that_defined => { cdata.get_expn_that_defined(def_id.index, tcx.sess) }
+    default_field => { cdata.get_default_field(def_id.index) }
     is_doc_hidden => { cdata.get_attr_flags(def_id.index).contains(AttrFlags::IS_DOC_HIDDEN) }
     doc_link_resolutions => { tcx.arena.alloc(cdata.get_doc_link_resolutions(def_id.index)) }
     doc_link_traits_in_scope => {