gccrs: attr: Register rustc_conversion_suggestion

This patch registers the 'rustc_conversion_suggestion' attribute. This
attribute is needed for compiling the alloc crate, so we register it as
a stub and the compiler will emit a warning when it encounters this
attribute.

gcc/rust/ChangeLog:

	* checks/errors/rust-builtin-attribute-checker.cc
	(attribute_checking_handlers): Add rustc_conversion_suggestion
	attribute to map.
	(check_valid_attribute_for_item): Add warning for new attribute.
	* util/rust-attribute-values.h (class Attributes): Add
	RUSTC_CONVERSION_SUGGESTION constexpr.
	* util/rust-attributes.cc (__definitions): Add
	BuiltinAttrDefinition for new attribute.

gcc/testsuite/ChangeLog:

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

Signed-off-by: Enes Cevik <enes@nsvke.com>