Sema: Skip missing async requirements of Obj-C protocols consistently.

Requirements of Obj-C protocols that take a completion handler get imported
with an `async` sibling requirement which can be fulfilled instead. Exactly
one of these two requirements should be fulfilled. When resolving value
witnesses ad-hoc, the compiler would only skip diagnosing a missing witness of
an async requirement if the sibling requirement were fulfilled and there were
zero viable matches. It shouldn't matter how many viable matches there are,
though, since the alternative requirement is already witnessed.

There is no test case yet because when this problem reproduces, an assertion
about resolving a witness after conformance checking is "complete" fails. A
more significant overhaul of this logic should come later, but short-term we
must fix source compatibility.

Resolves rdar://124145619

Author: tshortli

lib/Sema/TypeCheckProtocol.cpp

@@ -4391,11 +4391,12 @@ ConformanceChecker::resolveWitnessViaLookup(ValueDecl *requirement) {
     return ResolveWitnessResult::ExplicitFailed;
   }
 
+  if (!shouldRecordMissingWitness(Proto, Conformance, requirement))
+    return ResolveWitnessResult::Missing;
 
   if (!numViable) {
     // Save the missing requirement for later diagnosis.
-    if (shouldRecordMissingWitness(Proto, Conformance, requirement))
-      getASTContext().addDelayedMissingWitness(Conformance, {requirement, matches});
+    getASTContext().addDelayedMissingWitness(Conformance, {requirement, matches});
     return ResolveWitnessResult::Missing;
   }