Fix: infer type of async block with tail return expr

[Kohei316]

Fixes #17106
The infer_async_block method calls the infer_block method internally, which returns the never type without coercion when tail_expr is None and ctx.diverges is Diverges::Always.This is the reason for the bug in this issue.

rust-analyzer/crates/hir-ty/src/infer/expr.rs

Lines 1411 to 1413 in cfce2bb

	if this.diverges.is_always() {
	// we don't even make an attempt at coercion
	this.table.new_maybe_never_var()

This PR solves the bug by adding a process to coerce after calling infer_block method.

This code passes all the tests, including tests I added for this isuue, however, I am not sure if this solution is right. I think that this solution is an ad hoc solution. So, I would appreciate to have your review.
I apologize if I'm off the mark, but infer_async_block method should be rewritten to share code with the process of infering type of expr::Closure instead of the infer_block method. That way it will be closer to the infer process of rustc.

[Veykril]

I think it suffices to merely replace this by self.infer_return(tgt_expr)?

[Kohei316]

I do not fully understand these functions and may be misunderstanding, but I think that we cannot use this way. First, infer_return method only coerces and does not return a value. (This problem may be solved by changing infer return.) Next, the method internally calls infer_expr_inner, which internally calls infer_async_block. This results in an infinite loop.