Fix detection of __datasizeof with Clang. (#94174)

The `__has_extension` builtin macro is the same as __has_feature when
-pedantic-errors is specified, which means we don't get the right
information about __datasizeof being available.

Using __has_keyword (really !__is_identifier) will tell the truth
even when -pedantic-errors is specified.

This means we always have __datasizeof under Clang