checker, cgen: fix match branches return type ref mismatch, when return type exists interface or sumtype(fix #16203)

[shove70]

1. Fixed Interface with string instance accessing underlying value missing * dereference (incompatible operand types) error #16203
2. Add tests.

Because smartcast adds one level of ref to the interface type, special handling is required in the return type of the match expr branch.

interface Any {}

fn (any Any) echo() {
	r := match any {
		string { any }
		else { 'unsupported' }
	}
	println(r)
}

Any('hallo').echo()

outputs:

hallo

[shove70]

Ok, I'll rearrange it.

I just noticed that sumtype has some C happen issues too:

type Any = int | string

fn (any Any) echo() {
	a := 'abc'
	r := match any {
		string { &any }
		else { a }
	}
	println(r)
}

Any('hallo').echo()

Running code...
Can't run code. The server returned an error:
/tmp/v_60000/../../../../../../box/code.v:5: error: type mismatch in conditional expression (have 'struct string *' and 'struct string')
builder error: 
==================
C error. This should never happen.

This is a compiler bug, please report it using `v bug file.v`.

type Any = int | string

fn (any Any) echo() {
	a := 'abc'
	r := match any {
		string { any }
		else { &a }
	}
	println(r)
}

Any('hallo').echo()

Running code...
Can't run code. The server returned an error:
/tmp/v_60000/../../../../../../box/code.v:5: error: type mismatch in conditional expression (have 'struct string' and 'struct string *')
builder error: 
==================
C error. This should never happen.

This is a compiler bug, please report it using `v bug file.v`.

Looks like the sumtype and interface match issues need to be handled together, and I will

[spytheman]

Thank you @shove70 .

[shove70]

Thank you @shove70 .

done, review it again when you're free, 🎉 @spytheman

[spytheman]

Good work.