cmd/compile: named type constraints that are not interfaces are not automatically embedded in implicit interfaces

[griesemer]

Test case:

func f[P ~int]() {}
func g[P int]() {}

var _ = f[int] // this is ok
var _ = g[int] // <<< ERROR int is not an interface

Per the spec, the declaration and instantiation of g should be valid.

Not a release blocker because one can always manually wrap the constraint as in:

package p

func f[P ~int]() {}
func g[P interface{int}]() {} // wrap in interface{}

var _ = f[int] // this is ok
var _ = g[int] // ok now

cc: @findleyr

[gopherbot]

Change https://golang.org/cl/375737 mentions this issue: cmd/compile/internal/types2: wrap non-interface constraints into implicit interface