cmd/compile/internal/types2: mark gotypesalias as undocumented

CL 541737 added gotypesalias to control whether Alias types are used.
This setting is meant to use by end users through go/types. However,
types2 also uses it, but it's an internal package, causing bootstrap
failed because of unknown setting.

Marking the setting as undocumented in types2 fixes the problem.

Fixes #64106

Change-Id: If51a63cb7a21d9411cd9cf81bca2530c476d22f8
Reviewed-on: https://go-review.googlesource.com/c/go/+/542135
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Mauri de Souza Meneguzzo <mauri870@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>
Auto-Submit: Cuong Manh Le <cuong.manhle.vn@gmail.com>

src/cmd/compile/internal/types2/check.go

@@ -23,7 +23,7 @@ var nopos syntax.Pos
 const debug = false // leave on during development
 
 // gotypesalias controls the use of Alias types.
-var gotypesalias = godebug.New("gotypesalias")
+var gotypesalias = godebug.New("#gotypesalias")
 
 // exprInfo stores information about an untyped expression.
 type exprInfo struct {