Slightly clean up and improve testing for ObjC selectors

Author: DougGregor

Sources/SwiftParser/Attributes.swift

@@ -458,7 +458,7 @@ extension Parser {
             name: name,
             colon: nil,
             arena: self.arena))
-          continue
+          break
         }
 
         let (unexpectedBeforeColon, colon) = self.expect(.colon)

Tests/SwiftParserTest/Attributes.swift

@@ -63,6 +63,9 @@ final class AttributeTests: XCTestCase {
       """
       @objc(zeroArg)
       class A { }
+
+      @objc(:::::)
+      func f(_: Int, _: Int, _: Int, _: Int, _: Int) { }
       """
     )
   }