ExtDecl: do not create singleton unions when dealing with case types

Summary:
When dealing with case types that are singleton unions, ExtDecl would create an ExtDeclTypeDef with the `type_` field as `(|T)`. This creates problems down the line since this is not a parsable type. `(|T)` results from creating a singleton union `Tunion(T)` then applying the hackrs type printer.

That type printer currently prints singleton unions as `(|T)` instead of just `T`. The reason is explained in [this comment](https://www.internalfb.com/diff/D52491369?transaction_fbid=3812697109009854): the code was ported from the ocaml printer `Typing_print.Full`, which is also used for debugging.

We could fix the printer, and we do in the previous diff, but it might get broken again later as devs want to debug more stuff.

So in addition, we also avoid creating singleton unions altogether before printing.

Reviewed By: viratyosin

Differential Revision: D67335740

fbshipit-source-id: 8e7673e22e702177eb1ac0a0b724ec9d4215fbf1

hphp/hack/src/hackc/ffi_bridge/ext_decl.rs

@@ -130,13 +130,19 @@ pub fn get_file_typedefs(parsed_file: &ParsedFile<'_>, name: &str) -> Vec<ExtDec
                     (enum_typedef_visibility(*vis), extract_type_name(hint))
                 }
                 TypedefTypeAssignment::CaseType((variant, variants)) => {
-                    let mut hints = variants.iter().map(|v| v.0).collect::<Vec<_>>();
-                    hints.insert(0, variant.0);
+                    let mut hints;
+                    let ty_ = if variants.is_empty() {
+                        variant.0.1
+                    } else {
+                        hints = variants.iter().map(|v| v.0).collect::<Vec<_>>();
+                        hints.insert(0, variant.0);
+                        Ty_::Tunion(&hints)
+                    };
                     (
                         String::from("case_type"),
                         extract_type_name(&Ty(
                             &typing_reason::Reason::NoReason, // the position is ignored when printing the type
-                            Ty_::Tunion(&hints),
+                            ty_,
                         )),
                     )
                 }