gccrs: Add attributes checker

gccrs: Add attributes checker

The attribute checker is responsible for checking the validity of various
attributes including built-in ones. It is currently unfinished and will
receive some modifications, as well as become the host of some existing
code in the compiler which needs to be refactored. One of its
responsibilities is to make sure that arguments given to built-in
attributes are correct, or contain the correct type of information. This
visitor also checks that an attribute is allowed to be used in the current
particular context.

	gcc/rust/
	* util/rust-attributes.cc: New.
	* util/rust-attributes.h: New.