gccrs: Add new getters

gccrs: Add new getters

Some class members were not accessible from outside, which will be needed
for the new HIR dump.

gcc/rust/ChangeLog:

	* hir/tree/rust-hir-expr.h (AwaitExpr::get_awaited_expr): New.
	(AsyncBlockExpr::get_has_move, AsyncBlockExpr::get_block_expr): New.
	* hir/tree/rust-hir-item.h (Enum::get_where_clause): New.
	(Trait::get_where_clause, Trait::is_unsafe): New.
	(ExternalItem::get_visibility): New.

Signed-off-by: Marc Poulhiès <dkm@kataplop.net>