Fix fn front matter parsing ICE from invalid code.

This commit fixes an "unreachable code" ICE that results from parsing
invalid code where the compiler is expecting the next trait item
declaration in the middle of the previous trait item due to extra
closing braces.

src/libsyntax/parse/parser.rs

@@ -6575,7 +6575,12 @@ impl<'a> Parser<'a> {
             };
             (respan(self.prev_span, Constness::NotConst), unsafety, abi)
         };
-        self.expect_keyword(keywords::Fn)?;
+        if !self.eat_keyword(keywords::Fn) {
+            // It is possible for `expect_one_of` to recover given the contents of
+            // `self.expected_tokens`, therefore, do not use `self.unexpected()` which doesn't
+            // account for this.
+            if !self.expect_one_of(&[], &[])? { unreachable!() }
+        }
         Ok((constness, unsafety, asyncness, abi))
     }