[patch] Use __builtin_FILE and __builtin_LINE in assert implementation in C++

[Florian Weimer]

* Paul Pluzhnikov via Libc-alpha:

> diff --git a/assert/assert.h b/assert/assert.h
> index 72209bc5e7..4e0303db8d 100644
> --- a/assert/assert.h
> +++ b/assert/assert.h
> @@ -89,7 +89,8 @@ __END_DECLS
>  #  define assert(expr)                                                 \
>       (static_cast <bool> (expr)
>          \
>        ? void (0)                                                       \
> -      : __assert_fail (#expr, __FILE__, __LINE__, __ASSERT_FUNCTION))
> +      : __assert_fail (#expr, __builtin_FILE (), __builtin_LINE (),     \
> +                       __ASSERT_FUNCTION))
>  # elif !defined __GNUC__ || defined __STRICT_ANSI__
>  #  define assert(expr)                                                 \
>      ((expr)                                                            \

I think __builtin_FILE and __builtin_LINE are farily recent GCC/Clang
additions, so they need compiler version checks or a __has_builtin gate.

Ideally, we would use <source_location> here, but I don't think we can
include that from <assert.h>.

Thanks,
Florian