Revert r318556 "Loosen -Wempty-body warning"

It seems this somehow made -Wempty-body fire in some macro cases where
it didn't before, e.g.

  ../../third_party/ffmpeg/libavcodec/bitstream.c(169,5):  error: if statement has empty body [-Werror,-Wempty-body]
      ff_dlog(NULL, "new table index=%d size=%d\n", table_index, table_size);
      ^
  ../../third_party/ffmpeg\libavutil/internal.h(276,80):  note: expanded from macro 'ff_dlog'
  #   define ff_dlog(ctx, ...) do { if (0) av_log(ctx, AV_LOG_DEBUG, __VA_ARGS__); } while (0)
                                                                                 ^
  ../../third_party/ffmpeg/libavcodec/bitstream.c(169,5):  note: put the
  semicolon on a separate line to silence this warning

Reverting until this can be figured out.

> Do not show it when `if` or `else` come from macros.
> E.g.,
>
>     #define USED(A) if (A); else
>     #define SOME_IF(A) if (A)
>
>     void test() {
>       // No warnings are shown in those cases now.
>       USED(0);
>       SOME_IF(0);
>     }
>
> Patch by Ilya Biryukov!
>
> Differential Revision: https://reviews.llvm.org/D40185

llvm-svn: 318665