refactor all attribute functions in rust-compile-base.cc

[saeitoshi-10]

gcc/rust/ChangeLog:
* backend/rust-compile-base.cc (get_attributes): (get_trait_name):
* util/rust-attributes.cc (Attributes::get_attributes): (Attributes::get_trait_name):
* util/rust-attributes.h:

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Fixes #3291

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