Allow implied conditional conformances to marker protocols.

This improves source compatibility in cases where a marker
protocol is introduced after-the-fact (e.g., for `Error`),
and is safe because there's no runtime component to marker protocols.

Author: DougGregor

lib/Sema/TypeCheckProtocol.cpp

@@ -1962,7 +1962,8 @@ checkIndividualConformance(NormalProtocolConformance *conformance,
   }
 
   bool impliedDisablesMissingWitnessFixits = false;
-  if (conformance->getSourceKind() == ConformanceEntryKind::Implied) {
+  if (conformance->getSourceKind() == ConformanceEntryKind::Implied &&
+      !Proto->isMarkerProtocol()) {
     // We've got something like:
     //
     //   protocol Foo : Proto {}

test/attr/attr_marker_protocol.swift

@@ -51,3 +51,10 @@ func testNotOkay(a: Any) {
   func inner(p3: P3) { }
 }
 
+
+@_marker protocol P8 { }
+protocol P9: P8 { }
+
+// Implied conditional conformance to P8 is okay because P8 is a marker
+// protocol.
+extension Array: P9 where Element: P9 { }