Fix preprocessor checks for Clang builtins

Fix preprocessor checks for Clang builtins

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.

Backport from mainline
2019-07-02  Jonathan Wakely  <jwakely@redhat.com>

	* 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: r273832