[libc++] Add __assertion_handler to the modulemap

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

@@ -2285,6 +2285,7 @@ module std [system] {
   }
   module internal_assert {
     header "__assert"
+    header "__assertion_handler" // Generated via CMake
     export *
   }