gccrs: attributes: Start handling prelude_import properly

gccrs: attributes: Start handling prelude_import properly

This commit adds basic handling for the `#[prelude_import]` attribute,
without doing anything functionality wise.

gcc/rust/ChangeLog:

	* checks/errors/rust-feature-gate.cc (FeatureGate::visit): Add base
	feature gating for `#[feature(prelude_import)]`.
	* checks/errors/rust-feature-gate.h: Likewise.
	* checks/errors/rust-feature.cc (Feature::create): Likewise.
	* checks/errors/rust-feature.h: Likewise.
	* util/rust-attribute-values.h: Add base handling for `#[prelude_import]`
	attribute.
	* util/rust-attributes.cc: Likewise.

gcc/testsuite/ChangeLog:

	* rust/compile/prelude_import.rs: New test.