go: cmd/compile: local type declarations in generic code don't work

package main

func F[T1 any]() {
	type x struct{}
}
func main() {
}

$ go run -gcflags=-G=3 tmp1.go
# command-line-arguments
<unknown line number>: internal compiler error: cannot export DCLTYPE (50) node

This looks like a general problem that we can’t export/import local type declarations, including for inlining.