gccrs: expand: Convert last statement to tail expr

gccrs: expand: Convert last statement to tail expr

Some inner statements may be expanded to statements as well as a tail
expression, which should then be propagated to the parent tail
expression field.

gcc/rust/ChangeLog:

	* expand/rust-expand-visitor.cc (ExpandVisitor::visit): Change
	call to expand_inner_stmts.
	(expand_tail_expr): Change argument name and try to expand the
	last statement as a tail expression once all statements have
	been expanded.
	* expand/rust-expand-visitor.h: Change prototype to accept
	parent class.
	* ast/rust-expr.h: Add try_convert_last_stmt function prototype.

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