Expect extern fn with no body when parsing

Also add a test case for inserting a semicolon on extern fns.

Without this fix, we got an error like this:

    error: expected one of `->`, `where`, or `{`, found `}`
     --> chk.rs:3:1
      |
    2 |   fn foo()
      |      ---  - expected one of `->`, `where`, or `{`
      |      |
      |      while parsing this `fn`
    3 | }
      | ^ unexpected token

Since this is inside an extern block, you're required to write function
prototypes with no body. This fixes a regression, and adds a test case
for it.

compiler/rustc_parse/src/parser/item.rs

@@ -950,7 +950,7 @@ impl<'a> Parser<'a> {
         &mut self,
         force_collect: ForceCollect,
     ) -> PResult<'a, Option<Option<P<ForeignItem>>>> {
-        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: true };
+        let fn_parse_mode = FnParseMode { req_name: |_| true, req_body: false };
         Ok(self.parse_item_(fn_parse_mode, force_collect)?.map(
             |Item { attrs, id, span, vis, ident, kind, tokens }| {
                 let kind = match ForeignItemKind::try_from(kind) {