[Modules] textual headers in submodules never resolve their uses

When an include from a textual header is resolved, the textual header's submodule is used as the requesting module. The submodule's uses are resolved, but that doesn't work because only top level modules have uses, and only the top level module uses are used for checking uses in Module::directlyUses. ModuleMap::resolveUses to resolve the top level module instead of the submodule.

clang/lib/Lex/ModuleMap.cpp

@@ -1398,16 +1398,17 @@ bool ModuleMap::resolveExports(Module *Mod, bool Complain) {
 }
 
 bool ModuleMap::resolveUses(Module *Mod, bool Complain) {
-  auto Unresolved = std::move(Mod->UnresolvedDirectUses);
-  Mod->UnresolvedDirectUses.clear();
+  auto *Top = Mod->getTopLevelModule();
+  auto Unresolved = std::move(Top->UnresolvedDirectUses);
+  Top->UnresolvedDirectUses.clear();
   for (auto &UDU : Unresolved) {
-    Module *DirectUse = resolveModuleId(UDU, Mod, Complain);
+    Module *DirectUse = resolveModuleId(UDU, Top, Complain);
     if (DirectUse)
-      Mod->DirectUses.push_back(DirectUse);
+      Top->DirectUses.push_back(DirectUse);
     else
-      Mod->UnresolvedDirectUses.push_back(UDU);
+      Top->UnresolvedDirectUses.push_back(UDU);
   }
-  return !Mod->UnresolvedDirectUses.empty();
+  return !Top->UnresolvedDirectUses.empty();
 }
 
 bool ModuleMap::resolveConflicts(Module *Mod, bool Complain) {

clang/test/Modules/no-undeclared-includes.c

@@ -0,0 +1,31 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %clang_cc1 -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %t %t/no-undeclared-includes.c -verify
+
+//--- no-undeclared-includes.c
+// expected-no-diagnostics
+#include <assert.h>
+
+//--- assert.h
+#include <base.h>
+
+//--- base.h
+#ifndef base_h
+#define base_h
+
+
+
+#endif /* base_h */
+
+//--- module.modulemap
+module cstd [system] [no_undeclared_includes] {
+  use base
+  module assert {
+    textual header "assert.h"
+  }
+}
+
+module base [system] {
+  header "base.h"
+  export *
+}