Sema: implement requiresClass using a request evaluator.

Add the request `ProtocolRequiresClassRequest` to lazily determine if a
`ProtocolDecl` requires conforming types to be a class.

Note that using the request evaluator to compute `requiresClass` introduces
cycle errors for protocol declarations, where this computation didn't
previously emit diagnostics. For now, we'll allow duplicate diagnostics in this
case, with the eventual goal of removing explicitly checking for cycles
via `checkCircularity` (instead letting the request evaluator handle cycle
diagnostics).

lib/Serialization/Deserialization.cpp

@@ -3323,7 +3323,8 @@ public:
                                              None, /*TrailingWhere=*/nullptr);
     declOrOffset = proto;
 
-    proto->setRequiresClass(isClassBounded);
+    ctx.evaluator.cacheOutput(ProtocolRequiresClassRequest{proto},
+                              std::move(isClassBounded));
     proto->setExistentialTypeSupported(existentialTypeSupported);
 
     if (auto accessLevel = getActualAccessLevel(rawAccessLevel)) {