go/types, types2: better error message for cases of reverse type inference

[griesemer]

This is a reminder issue to follow up on feedback in https://go.dev/cl/501938 (#60688).

v9 func(string) []int = f5 // ERROR "type func(string) []int of variable in assignment does not match inferred type func(string) []string for func(P) []P"

Feedback (@findleyr):

I think a better error would be "type func(string) []int of variable v9 does not match inferred type func(string) []string for f5"

[gopherbot]

Change https://go.dev/cl/546456 mentions this issue: types2: better error message for cases of reverse type inference

[gopherbot]

Change https://go.dev/cl/548995 mentions this issue: go/types, types2: better error message for cases of reverse type inference

[gopherbot]

Change https://go.dev/cl/549055 mentions this issue: go/types, types2: flip message contents for for reverse type inference errors

[gopherbot]

Change https://go.dev/cl/549055 mentions this issue: go/types, types2: flip message contents for reverse type inference errors