[Bug c/71255] Implement #pragma may_alias

[mpolacek at gcc dot gnu.org]

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

--- Comment #10 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Well, so far my plan is to gather symbols in
#pragma GCC may_alias SYMBOL
to some vector of symbols and then when declaring a SYMBOL check if it's in the
vector and if so, apply the may_alias attribute.  Now, as Richi points out,
I'll probably have to introduce two variants:

#pragma GCC may_alias SYMBOL
#pragma GCC may_alias struct SYMBOL

The symbols in the vector would serve as a "tentative" forward declaration.

Do you see any problems with that?