refactor all attribute functions in rust-collect-lang-items.cc

[saeitoshi-10]

gcc/rust/ChangeLog:
* ast/rust-collect-lang-items.cc (get_lang_item_attr): "removed checker fn"
* util/rust-attributes.cc (Attributes::is_lang_item): "added fn"
* util/rust-attributes.h: "added fn"

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Addresses #3291
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[philberty]

LGTM