gccrs: ast: Add Tokenstream visitors for loop expressions

gccrs: ast: Add Tokenstream visitors for loop expressions

Add the implementation of tokenstream dump for multiple loop
expressions.

gcc/rust/ChangeLog:

	* ast/rust-ast-tokenstream.cc (TokenStream::visit): Add
	visitors.
	(TokenStream::visit_loop_common): Merge common loop code.
	* ast/rust-ast-tokenstream.h: Add function prototypes.

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