libstdc++: Simplify detection of built-in functions

libstdc++: Simplify detection of built-in functions

This fixes a regression affecting the Intel compiler. Because that
compiler defines __GNUC__ to match whatever version of GCC it finds on
the host system, it might claim to be a brand new GCC despite not
actually supporting all the built-ins that the latest GCC supports. This
means the config checks for __GNUC__ don't work. Most recently this
broke when r11-3569-g73ae6eb572515ad627b575a7fbdfdd47a4368e1c switched
us from using __is_same_as to __is_same when __GNUC__ >= 11.

Because __has_builtin is supported by all of GCC, Clang, and Intel we can
use that to reliably detect whether a given built-in is supported,
instead of hardcoding anything based on __GNUC__. The big caveat is
that for versions of Clang <= 9.0.0 and for (as far as I can tell) all
released versions of Intel icc, __has_builtin only evaluates to true for
built-ins with a name starting "__builtin_". For __is_aggregate,
__is_same, and __has_unique_object_representations it's necessary to use
__is_identifier to check if it's a valid identifeir token instead.

The solution used in this patch is to define _GLIBCXX_HAS_BUILTIN and
use that instead of using __has_builtin directly. For compilers that
define __is_identifier as well as __has_builtin we use both, so that if
__has_builtin evaluates to false we try again using !__is_identifier.

libstdc++-v3/ChangeLog:

	* include/bits/c++config (_GLIBCXX_HAS_BUILTIN): Define macro to
	work around different implementations of __has_builtin.
	(_GLIBCXX_HAVE_BUILTIN_HAS_UNIQ_OBJ_REP)
	(_GLIBCXX_HAVE_BUILTIN_IS_AGGREGATE)
	(_GLIBCXX_HAVE_BUILTIN_IS_CONSTANT_EVALUATED)
	(_GLIBCXX_HAVE_BUILTIN_IS_SAME, _GLIBCXX_HAVE_BUILTIN_LAUNDER):
	Define using _GLIBCXX_HAS_BUILTIN.