[Bug c/71255] Implement #pragma may_alias

[mpolacek at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71255

--- Comment #15 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Yeah, only the C++ side was changed.  I think it's wrong that we reject the
testcase in Comment 14 in C (I have a fix for that).

But even with that fixed we still need the new #pragma because of the second
testcase in Comment 13, right?