[Async CC] Don't lookup known archetype conformance.

Previously, when saving NecessaryBindings for an async function, if a
type that was passed-in happened to be an archetype, a lookup for that
type's conformance would always be done.  However, that lookup was not
always necessary or possible such as in the case where both the metadata
and the witness table were provided to the function in an async context.
Here, that is fixed by using the conformance that was seen when
constructing the NecessaryBindings if one is available.

rdar://problem/72397303

Author: nate-chandler

lib/IRGen/GenProto.cpp

@@ -2712,12 +2712,14 @@ static void save(const NecessaryBindings &bindings, IRGenFunction &IGF,
       [&](GenericRequirement requirement) -> llvm::Value * {
         CanType type = requirement.TypeParameter;
         if (auto protocol = requirement.Protocol) {
-          if (auto archetype = dyn_cast<ArchetypeType>(type)) {
+          CanArchetypeType archetype;
+          ProtocolConformanceRef conformance =
+              bindings.getConformance(requirement);
+          if ((archetype = dyn_cast<ArchetypeType>(type)) && !conformance) {
             auto wtable =
                 emitArchetypeWitnessTableRef(IGF, archetype, protocol);
             return transform(requirement, wtable);
           } else {
-            auto conformance = bindings.getConformance(requirement);
             auto wtable = emitWitnessTableRef(IGF, type, conformance);
             return transform(requirement, wtable);
           }

validation-test/compiler_crashers_2_fixed/rdar72397303.swift

@@ -0,0 +1,10 @@
+// RUN: %target-swift-frontend %s -emit-ir -enable-experimental-concurrency
+
+protocol P {
+  func f<T>() async throws -> T?
+}
+extension P {
+  func f<T>() async throws -> T? { nil }
+}
+class X: P {}
+