modulemap: add arm64 intrinsics header

The header was missing from the modulemap definition, resulting in a
breakage for Swift with recent Windows SDK headers.

Author: Steelskin

clang/lib/Headers/module.modulemap

@@ -35,6 +35,13 @@ module _Builtin_intrinsics [system] [extern_c] {
     }
   }
 
+  explicit module arm64 {
+    requires arm64
+
+    header "arm64intr.h"
+    export *
+  }
+
   explicit module intel {
     requires x86
     export *