Clang seems to define built-ins that start with "__builtin_" as
non-keywords, which means that we need to use __has_builtin to detect
them, not __is_identifier. The built-ins that don't start with
"__builtin_" are keywords, and can only be detected using
__is_identifier and not by __has_builtin.
* include/bits/c++config (_GLIBCXX_HAVE_BUILTIN_LAUNDER)
(_GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED): Use __has_builtin
instead of __is_identifier to detect Clang support.
From-SVN: r272931
