[Dependency Scanning] Warn, instead of fail, when a Swift dependency query only finds modules built for incompatible target

We have adopters who are relying on directly importing the underlying Clang module in the presence of incompatible Swift modules.

Resolves rdar://162549210

lib/DependencyScan/ModuleDependencyScanner.cpp

@@ -2088,6 +2088,19 @@ void ModuleDependencyIssueReporter::diagnoseFailureOnOnlyIncompatibleCandidates(
   if (candidates.empty())
     return;
 
+  // FIXME: There are known cases where clients are relying on
+  // loading the underlying Clang module in the presence of a Swift
+  // module which is lacking the required target-specific variant,
+  // such as MacCatalyst. Eventually, we should pursue making this
+  // an error as well.
+  if (llvm::all_of(candidates, [](auto &incompatibleCandidate) {
+        return incompatibleCandidate.incompatibilityReason ==
+            SwiftModuleScannerQueryResult::BUILT_FOR_INCOMPATIBLE_TARGET;
+      })) {
+    warnOnIncompatibleCandidates(moduleImport.importIdentifier, candidates);
+    return;
+  }
+
   diagnoseModuleNotFoundFailure(moduleImport, cache, dependencyOf,
                                 /* resolvingSerializedSearchPath */ std::nullopt,
                                 candidates);