[libc++] Automatically detect whether RTTI is enabled

Instead of detecting it automatically but also allowing for the setting
to be specified explicitly, always detect whether exceptions are enabled
based on whether -fno-rtti (or equivalent) is used. It's less confusing
to have a single way of tweaking that knob.

This change follows the lead of 71d88cebfb42.

utils/gn/secondary/libcxx/src/BUILD.gn

@@ -102,7 +102,6 @@ config("cxx_config") {
     } else {
       cflags_cc += [ "-fno-rtti" ]
     }
-    defines += [ "_LIBCPP_NO_RTTI" ]
   }
 }