gccrs: Document proc macro token tree indices

gccrs: Document proc macro token tree indices

Multiple references to procedural macro token trees were left as magic
number in the code. This commit introduces some constexpr for those along
some explanation for the selected value.

gcc/rust/ChangeLog:

	* backend/rust-compile-base.cc (get_attributes): Add documentation for
	indices 3 and 4.
	(get_trait_name): Add documentation for index 1.

Signed-off-by: Pierre-Emmanuel Patry <pierre-emmanuel.patry@embecosm.com>