[clang][modules] Built-in modules are not correctly enabled for Mac C…

…atalyst (llvm#104872)

Mac Catalyst is the iOS platform, but it builds against the macOS SDK
and so it needs to be checking the macOS SDK version instead of the iOS
one. Add tests against a greater-than SDK version just to make sure this
works beyond the initially supporting SDKs.

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -2953,7 +2953,15 @@ static bool sdkSupportsBuiltinModules(
   case Darwin::MacOS:
     return SDKVersion >= VersionTuple(15U);
   case Darwin::IPhoneOS:
-    return SDKVersion >= VersionTuple(18U);
+    switch (TargetEnvironment) {
+    case Darwin::MacCatalyst:
+      // Mac Catalyst uses `-target arm64-apple-ios18.0-macabi` so the platform
+      // is iOS, but it builds with the macOS SDK, so it's the macOS SDK version
+      // that's relevant.
+      return SDKVersion >= VersionTuple(15U);
+    default:
+      return SDKVersion >= VersionTuple(18U);
+    }
   case Darwin::TvOS:
     return SDKVersion >= VersionTuple(18U);
   case Darwin::WatchOS:

clang/test/Driver/Inputs/MacOSX15.1.sdk/SDKSettings.json

@@ -0,0 +1 @@
+{"Version":"15.1", "MaximumDeploymentTarget": "15.1.99"}

clang/test/Driver/darwin-builtin-modules.c

@@ -8,5 +8,8 @@
 
 // RUN: %clang -isysroot %S/Inputs/MacOSX15.0.sdk -target x86_64-apple-macos14.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
 // RUN: %clang -isysroot %S/Inputs/MacOSX15.0.sdk -target x86_64-apple-macos15.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// RUN: %clang -isysroot %S/Inputs/MacOSX15.0.sdk -target x86_64-apple-ios18.0-macabi -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// RUN: %clang -isysroot %S/Inputs/MacOSX15.1.sdk -target x86_64-apple-macos15.1 -darwin-target-variant x86_64-apple-ios18.1-macabi -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
+// RUN: %clang -isysroot %S/Inputs/MacOSX15.1.sdk -target x86_64-apple-ios18.1-macabi -darwin-target-variant x86_64-apple-macos15.1 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
 // RUN: %clang -isysroot %S/Inputs/DriverKit23.0.sdk -target arm64-apple-driverkit23.0 -### %s 2>&1 | FileCheck --check-prefix=CHECK_FUTURE %s
 // CHECK_FUTURE-NOT: -fbuiltin-headers-in-system-modules