[Bug c/106929] declaration in switch-case doesn't fail since GCC 11

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106929

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
This changed with r11-4813 and is intentional:

C Parser: Implement mixing of labels and code.

Implement mixing of labels and code as adopted for C2X
and process some std-attributes on labels.