[Bug c/126264] attribute error is ignored with invalid param

[jg at jguk dot org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126264

--- Comment #5 from Jonny Grant <jg at jguk dot org> ---
Could allowing attribute error and attribute warning to not have any
string-literal be considered.

That is consider allowing these. static_assert() does not require a
string-literal message.

void _fail() __attribute__ ((error));
void _fail() __attribute__ ((error()));

void _warn() __attribute__ ((warning));
void _warn() __attribute__ ((warning()));