Work around issue with inherited Sendable conformance lookup.

Implicit synthesis of `Sendable` conformances for global actor-isolated
class types interacts poorly with the conformance lookup table's
attempt at modeling inherited conformances, so a `Sendable` conformance
will get created and inherited, but is then "missing" when we try to
form the actual inherited conformance.

The proper fix for this issue is likely to eliminate the modeling of
inherited conformances within the conformance lookup table, which
introduces a lot of redundance and, apparently, some bugs. For now,
patch over the issue to work around a crash.

Narrowly works around rdar://81700570.

Author: DougGregor

lib/AST/ConformanceLookupTable.cpp

@@ -798,7 +798,30 @@ DeclContext *ConformanceLookupTable::getConformingContext(
     // Grab the superclass entry and continue searching for a
     // non-inherited conformance.
     // FIXME: Ambiguity detection and resolution.
-    entry = superclassDecl->ConformanceTable->Conformances[protocol].front();
+    const auto &superclassConformances =
+        superclassDecl->ConformanceTable->Conformances[protocol];
+    if (superclassConformances.empty()) {
+      assert(protocol->isSpecificProtocol(KnownProtocolKind::Sendable));
+
+      // Go dig for a superclass that does conform to Sendable.
+      // FIXME: This is a hack because the inherited conformances aren't
+      // getting updated properly.
+      Type classTy = nominal->getDeclaredInterfaceType();
+      ModuleDecl *module = nominal->getParentModule();
+      do {
+        Type superclassTy = classTy->getSuperclassForDecl(superclassDecl);
+        if (superclassTy->is<ErrorType>())
+          return nullptr;
+        auto inheritedConformance = module->lookupConformance(
+            superclassTy, protocol);
+        if (inheritedConformance)
+          return superclassDecl;
+      } while ((superclassDecl = superclassDecl->getSuperclassDecl()));
+
+      return nullptr;
+    }
+
+    entry = superclassConformances.front();
     nominal = superclassDecl;
   }