[libc++] Add __assertion_handler to the modulemap (#131031)

That header is generated via CMake, but it is nonetheless present in the
final installation, so it should be covered by the modulemap.

rdar://131418726

Author: ldionne

libcxx/include/module.modulemap

@@ -2317,6 +2317,10 @@ module std [system] {
     header "__assert"
     export *
   }
+  module assertion_handler {
+    header "__assertion_handler" // generated via CMake
+    export *
+  }
 
   module undef_macros {
     textual header "__undef_macros"