Teach "find imports" to equate overlay modules with their underlying modules

The operation that finds the best import for a given declaration was
treating an overload module as being distinct from its underlying
module, even though they both have the same name and are imported
together. Teach it to treat those modules as equivalent, so we
correctly identify the right import declaration for something that
comes from the underlying module.

Fixes rdar://129401319.

Author: DougGregor

lib/AST/Module.cpp

@@ -2668,9 +2668,11 @@ ImportDeclRequest::evaluate(Evaluator &evaluator, const SourceFile *sf,
   auto &ctx = sf->getASTContext();
   auto imports = sf->getImports();
 
+  auto mutModule = const_cast<ModuleDecl *>(module);
   // Look to see if the owning module was directly imported.
   for (const auto &import : imports) {
-    if (import.module.importedModule == module)
+    if (import.module.importedModule
+            ->isSameModuleLookingThroughOverlays(mutModule))
       return import;
   }
 
@@ -2679,7 +2681,8 @@ ImportDeclRequest::evaluate(Evaluator &evaluator, const SourceFile *sf,
   for (const auto &import : imports) {
     auto &importSet = importCache.getImportSet(import.module.importedModule);
     for (const auto &transitive : importSet.getTransitiveImports()) {
-      if (transitive.importedModule == module) {
+      if (transitive.importedModule
+              ->isSameModuleLookingThroughOverlays(mutModule)) {
         return import;
       }
     }

test/Concurrency/predates_concurrency_import_foundation_darwin.swift

@@ -0,0 +1,12 @@
+// RUN: %empty-directory(%t)
+
+// RUN: %target-swift-frontend -swift-version 6 -I %t %s -emit-sil -o /dev/null -verify -parse-as-library
+
+// REQUIRES: OS=macosx
+
+import Foundation
+@preconcurrency import Darwin
+
+func mach_task_self() -> mach_port_t {
+    return mach_task_self_
+}