go/types, types2: assignment instead of inference error for invalid generic function assignment

[griesemer]

type String string

func g[P any](P, string) {}

var f func(int, String) = g

produces the error (playground):

cannot use g (value of type func(int, string)) as func(int, String) value in variable declaration

but String and string are not identical and thus inference should have failed with a different error:

type func[P any](P, string) of g does not match func(int, String)

which is what is reported when g is passed to a function with a func(int, String) argument (playground).

Marking for 1.21 because the fix should not be too hard, but ok to defer to 1.22.

@findleyr for visibilty

[gopherbot]

Change https://go.dev/cl/501938 mentions this issue: go/types, types2: use correct inference approach in assignments of generic functions