interp: preserve concrete type when converting type to interface

This allows to fix the reassignment of an non empty interface value.
Before, reassignment was limited to empty interfaces.
    
Fixes #1138.

_test/issue-1136.go

@@ -5,8 +5,18 @@ import (
 	"io"
 )
 
+type T struct {
+	r io.Reader
+}
+
+func (t *T) Read(p []byte) (n int, err error) { n, err = t.r.Read(p); return }
+
 func main() {
 	x := io.LimitedReader{}
 	y := io.Reader(&x)
-	fmt.Println(y)
+	y = &T{y}
+	fmt.Println(y.Read([]byte("")))
 }
+
+// Output:
+// 0 EOF

interp/cfg.go

@@ -930,9 +930,12 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 					if !c1.typ.implements(c0.typ) {
 						err = n.cfgErrorf("type %v does not implement interface %v", c1.typ.id(), c0.typ.id())
 					}
-					// Pass value as is
+					// Convert type to interface while keeping a reference to the original concrete type.
+					// besides type, the node value remains preserved.
 					n.gen = nop
-					n.typ = c1.typ
+					t := *c0.typ
+					n.typ = &t
+					n.typ.val = c1.typ
 					n.findex = c1.findex
 					n.level = c1.level
 					n.val = c1.val