Polymorphic qualifier substitution in member references

While working on #2876 I've noticed that polymorphic qualifier substitution does not seem to work for member references.

Take:

import java.util.function.Supplier;
import testlib.util.Encrypted;
import testlib.util.PolyEncrypted;

class PolyMemberRef {
  @SuppressWarnings("return.type.incompatible") // impl
  @PolyEncrypted String id(@PolyEncrypted PolyMemberRef this) { return this.toString(); }

  void giveUnq(Supplier<String> s) {}
  void giveEnc(Supplier<@Encrypted String> s) {}

  void foo(PolyMemberRef u, @Encrypted PolyMemberRef e) {
    giveUnq(u::id);
    giveEnc(e::id);
  }
}

Run something like: ./checker/bin-devel/javac -cp framework/build/classes/java/test -processor org.checkerframework.common.subtyping.SubtypingChecker -Aquals=testlib.util.Encrypted,testlib.util.PolyEncrypted,org.checkerframework.common.subtyping.qual.Unqualified PolyMemberRef.java

And you'll get an error:

PolyMemberRef.java:14: error: [methodref.return.invalid] Incompatible return type
    giveEnc(e::id);
            ^
  Method
    String id(PolyMemberRef this) in PolyMemberRef
  is not a valid method reference for
    @Encrypted String get(Supplier<@Encrypted String> this) in java.util.function.Supplier
  found   : String
  required: @Encrypted String
1 error

The receiver used in the member reference should be used to resolve the polymorphic qualifier.

Start by running the provided javac command with the PolyMemberRef.java reproducer and confirm that the encrypted member reference is rejected. Trace the checker’s member-reference handling and polymorphic qualifier substitution, then verify that using the encrypted receiver permits giveEnc(e::id) while the unqualified case still passes.