clang-format: Fix corner case in ObjC interface definitions.

clang-format: Fix corner case in ObjC interface definitions.

In
  @implementation ObjcClass
  - (void)method;
  {
  }
  @end
the ObjC compiler seems to accept the superfluous comma after "method",
but clang-format used to assert on the subsequent "{".

This fixes llvm.org/PR16604.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189453 91177308-0d34-0410-b5e6-96231b3b80d8