go/types: respect IgnoreFuncBodies for function literals

This is a 1:1 port of CL 313650 to go/types.

Change-Id: Iec01ac2831f21162d9977a139549e081ee769f90
Reviewed-on: https://go-review.googlesource.com/c/go/+/314631
Trust: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

src/go/types/expr.go

@@ -1085,6 +1085,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 
 	case *ast.FuncLit:
 		if sig, ok := check.typ(e.Type).(*Signature); ok {
+			if !check.conf.IgnoreFuncBodies && e.Body != nil {
 				// Anonymous functions are considered part of the
 				// init expression/func declaration which contains
 				// them: use existing package-level declaration info.
@@ -1097,6 +1098,7 @@ func (check *Checker) exprInternal(x *operand, e ast.Expr, hint Type) exprKind {
 				check.later(func() {
 					check.funcBody(decl, "<function literal>", sig, e.Body, iota)
 				})
+			}
 			x.mode = value
 			x.typ = sig
 		} else {

src/go/types/stmt.go

@@ -14,6 +14,10 @@ import (
 )
 
 func (check *Checker) funcBody(decl *declInfo, name string, sig *Signature, body *ast.BlockStmt, iota constant.Value) {
+	if check.conf.IgnoreFuncBodies {
+		panic("internal error: function body not ignored")
+	}
+
 	if trace {
 		check.trace(body.Pos(), "--- %s: %s", name, sig)
 		defer func() {