[modules] allow use of ptrauth module from no_undeclared_includes sys…

…tem modules (#88432)

Co-authored-by: Alex Lorenz <arphaman@gmail.com>

clang/lib/Basic/Module.cpp

@@ -305,6 +305,10 @@ bool Module::directlyUses(const Module *Requested) {
   if (Requested->fullModuleNameIs({"_Builtin_stddef", "max_align_t"}) ||
       Requested->fullModuleNameIs({"_Builtin_stddef_wint_t"}))
     return true;
+  // Darwin is allowed is to use our builtin 'ptrauth.h' and its accompanying
+  // module.
+  if (!Requested->Parent && Requested->Name == "ptrauth")
+    return true;
 
   if (NoUndeclaredIncludes)
     UndeclaredUses.insert(Requested);

clang/test/Modules/Inputs/ptrauth-include-from-darwin/module.modulemap

@@ -0,0 +1,8 @@
+module libc [no_undeclared_includes] {
+  module stddef { header "stddef.h" export * }
+}
+
+module ptrauth {
+  header "ptrauth.h"
+  export *
+}

clang/test/Modules/Inputs/ptrauth-include-from-darwin/ptrauth.h

@@ -0,0 +1 @@
+void foo();

clang/test/Modules/Inputs/ptrauth-include-from-darwin/stddef.h

@@ -0,0 +1 @@
+@import ptrauth;

clang/test/Modules/ptrauth-include-from-darwin.m

@@ -0,0 +1,6 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -fmodules-cache-path=%t -fmodules -fimplicit-module-maps -I %S/Inputs/ptrauth-include-from-darwin %s -verify
+// expected-no-diagnostics
+
+@import libc;
+void bar() { foo(); }