Fixes ICE when using reexported unit-like structs

Fixes that unit-like structs cannot be used if they are reexported and
used in another crate. The compiler fails with an ICE, because unit-like
structs are exported as DefFn and the expression `UnitStruct` is
interpreted as function pointer instead of a call to the constructor.

To resolve this ambiguity tuple-like struct constructors are now exported
as CtorFn. When `rustc::metadata::decoder` finds a CtorFn it sets a new
flag `is_ctor` in DefFn to true.

Relevant changes are in `rustc::metadata::{encoder, decoder}` and in
`rustc::middle::ty`.

Closes #12660 and #16973.

src/librustdoc/clean/mod.rs

@@ -2078,7 +2078,7 @@ fn resolve_type(cx: &DocContext, path: Path,
 
 fn register_def(cx: &DocContext, def: def::Def) -> ast::DefId {
     let (did, kind) = match def {
-        def::DefFn(i, _) => (i, TypeFunction),
+        def::DefFn(i, _, _) => (i, TypeFunction),
         def::DefTy(i, false) => (i, TypeTypedef),
         def::DefTy(i, true) => (i, TypeEnum),
         def::DefTrait(i) => (i, TypeTrait),