[Modules] textual headers in submodules never resolve their uses (#69651)

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.

Author: ian-twilightcoder

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 *
+}