[feature availability] Add feature-availability.h to module map

rdar://152588813

Author: ahatanaka

clang/lib/Headers/module.modulemap

@@ -332,6 +332,11 @@ module ptrauth {
   export *
 }
 
+module feature_availability {
+  header "feature-availability.h"
+  export *
+}
+
 /* TO_UPSTREAM(BoundsSafety) ON */
 module ptrcheck {
   header "ptrcheck.h"

clang/test/Modules/feature-availability.c

@@ -1,6 +1,7 @@
 // RUN: %clang_cc1 -triple arm64-apple-macosx -fmodules %S/Inputs/feature-availability/module.modulemap -fmodule-name=Feature1 -emit-module -o %t/feature1.pcm
 // RUN: %clang_cc1 -triple arm64-apple-macosx -fmodules %S/Inputs/feature-availability/module.modulemap -fmodule-name=Feature2 -fmodule-file=%t/feature1.pcm -emit-module -o %t/feature2.pcm
 // RUN: %clang_cc1 -triple arm64-apple-macosx -fmodules -fmodule-file=%t/feature2.pcm -I %S/Inputs/feature-availability -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple arm64-apple-macosx -fmodules -fimplicit-module-maps -fmodules-cache-path=%t -I %S/Inputs/feature-availability -Werror=non-modular-include-in-module -emit-llvm -o - %s | FileCheck %s
 
 #include <feature-availability.h>
 #include "feature2.h"