[Issue 22598] importC: Add support for __extension__ keyword

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=22598

--- Comment #4 from Walter Bright <bugzilla at digitalmars.com> ---
(In reply to dave287091 from comment #3)
> It’s a gnu statement expression. See
> https://gcc.gnu.org/onlinedocs/gcc/Statement-Exprs.html. It’s also supported
> by clang.

Too bad there's no grammar for it.

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