Call normalizeOpaque in window of vulnerability

It turns out we get confused when we call normalizeOpaque as part of normal
completion during computeDenot. Calling normalizeOpaque in the window
of vulnerability defined where a type is Opaque but not yet Deferred
fixes the problem. Before the following tests failed
when compiled from-tasty:

    tests/run/implicit-specifity.scala failed
    tests/pos/i5720.scala failed
    tests/pos/toplevel-opaque-xm failed

Author: odersky

compiler/src/dotty/tools/dotc/core/Types.scala

@@ -1883,8 +1883,10 @@ object Types {
             finish(memberDenot(symd.initial.name, allowPrivate = false))
           else if (prefix.isArgPrefixOf(symd))
             finish(argDenot(sym.asType))
-          else if (infoDependsOnPrefix(symd, prefix))
+          else if (infoDependsOnPrefix(symd, prefix)) {
+            if (!symd.isClass && symd.is(Opaque, butNot = Deferred)) symd.normalizeOpaque()
             finish(memberDenot(symd.initial.name, allowPrivate = symd.is(Private)))
+          }
           else
             finish(symd.current)
       }