go/types, types2: type-checker internal type printing doesn't print embedded fields that are alias types correctly

[griesemer]

Given

type E = map[string]int
type S struct {
   E
}

the struct S will be printed with an embedded field named map[string]int because we have lost information about its alias name.

This affects debugging and test cases, e.g., src/go/internal/gccgoimporter/importer_test.go:97 wants "type S struct{b int; A2" but we only have "type S struct{b int; map[Y]Z}".

The relevant printing code is in go/types/typestring.go, function writeType.

[griesemer]

If we solve the type alias related issues (by introducing an explicit type alias node), this issue would probably be fixed as well.

[gopherbot]

Change https://golang.org/cl/293962 mentions this issue: [dev.typeparams] go/types, types2: revert fancy struct printing (fixes x/tools tests)