reflect: allow creation of recursive types

Introduce two new functions, Named and Bind, to the reflect API to
create possibly recursive, named types.

Fixes golang#39528.

src/cmd/compile/internal/gc/reflect.go

@@ -812,6 +812,7 @@ const (
 	tflagExtraStar     = 1 << 1
 	tflagNamed         = 1 << 2
 	tflagRegularMemory = 1 << 3
+	tflagIncomplete    = 1 << 4
 )
 
 var (

src/cmd/link/internal/ld/decodesym.go

@@ -24,8 +24,9 @@ import (
 //	reflect/type.go
 //	runtime/type.go
 const (
-	tflagUncommon  = 1 << 0
-	tflagExtraStar = 1 << 1
+	tflagUncommon   = 1 << 0
+	tflagExtraStar  = 1 << 1
+	tflagIncomplete = 1 << 4
 )
 
 func decodeInuxi(arch *sys.Arch, p []byte, sz int) uint64 {

src/internal/reflectlite/type.go

@@ -142,6 +142,10 @@ const (
 	// tflagRegularMemory means that equal and hash functions can treat
 	// this type as a single region of t.size bytes.
 	tflagRegularMemory tflag = 1 << 3
+
+	// tflagIncomplete means this type was created by Named and Bind has not
+	// been called on it yet.
+	tflagIncomplete tflag = 1 << 4
 )
 
 // rtype is the common implementation of most values.