Golang go/types: 'under' panics on Alias type [1.23 backport]

@findleyr requested issue #68877 to be considered for backport to the next 1.23 minor release.

@gopherbot, please create a backport issue for Go 1.23: this is a regression in the type checker when run with GODEBUG=gotypesalias=1, the new default.

Comment From: gopherbot

Change https://go.dev/cl/606656 mentions this issue: [release-branch.go1.23] go/types, types2: Named.cleanup must also handle *Alias types

Comment From: gopherbot

Closed by merging CL 606656 (commit 9166d2feec7c6fe5e4db802cca162745dac93488) to release-branch.go1.23.