gccrs: ast: Refactor SimplePath attribute name

gccrs: ast: Refactor SimplePath attribute name

The getter name was overly complicated and long, this could be
refactored to a shorter name.

gcc/rust/ChangeLog:

	* ast/rust-ast-tokenstream.cc (TokenStream::visit): Change
	getter name.
	* ast/rust-ast.cc (SimplePath::as_string): Change attribute
	name.
	* ast/rust-ast.h (class SimplePath): Refactor attribute name and
	change getter name.

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