[PATCH] Fix preprocessor checks for Clang builtins

[Jonathan Wakely <jwakely at redhat dot com>]

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.

Tested x86_64-linux, and verified that with Clang trunk both
std::is_constant_evaluated and std::launder get defined now.

Attachment: patch.txt
Description: Text document