Fix #4891: Make scala.quoted.Expr covariant

Author: nicolasstucki

library/src/scala/quoted/Expr.scala

@@ -2,7 +2,7 @@ package scala.quoted
 
 import scala.runtime.quoted.Unpickler.Pickled
 
-sealed abstract class Expr[T] {
+sealed abstract class Expr[+T] {
   final def unary_~ : T = throw new Error("~ should have been compiled away")
 
   /** Evaluate the contents of this expression and return the result.

tests/pos/i4891.scala

@@ -0,0 +1,5 @@
+import scala.quoted._
+
+object Test {
+  def foo: Expr[Option[String]] = '(None)
+}