[Parser][ObjC] Fix crash on nested top-level block with better recovery path

Delay consuming tokens until we are certain that the next token is not top
level block. Otherwise we bail out as if we saw an @end for better diagnostic
and recovery.

Fixes https://github.com/llvm/llvm-project/issues/64065.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D156277.