Avoid stringer-related deadlocks without adding ISGOMOCK

A deadlock related to controller calling Stringer on mocks themselves
was revealed in uber-go#116. uber-go#114 solved this deadlock by adding a generated
`ISGOMOCK()` method to all generated mocks, and then checking for it
before calling `.String()` on arguments.

This reveals an exported method on each generated mock that:
* Bloats the generated code
* Can be taken dependency on in strange ways via Hyrum's Law
* Technically opens up another route for naming collision.

This PR attempts to clean up this type of check by instead generating
an unexported field in generated mock structs instead, and checks for it using reflect.
This hides this implementation detail of gomock/mockgen better,
and produces less generated bloat.

Ref: uber-go#116

gomock/string.go

@@ -1,25 +1,36 @@
 package gomock
 
-import "fmt"
-
-type mockInstance interface {
-	ISGOMOCK() struct{}
-}
-type mockedStringer interface {
-	fmt.Stringer
-	mockInstance
-}
+import (
+	"fmt"
+	"reflect"
+)
 
 // getString is a safe way to convert a value to a string for printing results
 // If the value is a a mock, getString avoids calling the mocked String() method,
 // which avoids potential deadlocks
 func getString(x any) string {
-	switch v := x.(type) {
-	case mockedStringer:
-		return fmt.Sprintf("%T", v)
-	case fmt.Stringer:
-		return v.String()
-	default:
-		return fmt.Sprintf("%v", v)
+	if isGeneratedMock(x) {
+		return fmt.Sprintf("%T", x)
+	}
+	if s, ok := x.(fmt.Stringer); ok {
+		return s.String()
+	}
+	return fmt.Sprintf("%v", x)
+}
+
+// isGeneratedMock checks if the given type has a "isgomock" field,
+// indicating it is a generated mock.
+func isGeneratedMock(x any) bool {
+	typ := reflect.TypeOf(x)
+	if typ == nil {
+		return false
+	}
+	if typ.Kind() == reflect.Ptr {
+		typ = typ.Elem()
+	}
+	if typ.Kind() != reflect.Struct {
+		return false
 	}
+	_, isgomock := typ.FieldByName("isgomock")
+	return isgomock
 }