Use many __attribute__s with clang.

Use many __attribute__s with clang.

* m4/gnulib-common.m4 (_Noreturn): Use __attribute__ __noreturn__ also
on clang.
* lib/cdefs.h (__glibc_clang_has_attribute, __glibc_clang_has_builtin):
New macros.
(__THROW, __THROWNL, __NTH, __NTHNL): Use __attribute__ __nothrow__ also
on clang.
(__warndecl, __warnattr, __errordecl): Use __attribute__ __diagnose_if__
also on older clang versions.
(__attribute__): Don't define to empty on clang.
(__attribute_malloc__): Use __attribute__ __malloc__ also on clang.
(__attribute_pure__): Use __attribute__ __pure__ also on clang.
(__attribute_const__): Use __attribute__ __const__ also on clang.
(__attribute_used__): Use __attribute__ __used__ also on clang.
(__attribute_noinline__): Use __attribute__ __noinline__ also on clang.
(__attribute_deprecated__): Use __attribute__ __deprecated__ also on
clang.
(__attribute_format_arg__): Use __attribute__ __format_arg__ also on
clang.
(__attribute_format_strfmon__): Use __attribute__ __format__ __strfmon__
also on clang.
(__nonnull): Use __attribute__ __nonnull__ also on clang.
(__attribute_warn_unused_result__): Use __attribute__
__warn_unused_result__ also on clang.
(__always_inline): Use __attribute__ __always_inline__ also on clang.
(__attribute_artificial__): Use __attribute__ __artificial__ also on
clang >= 7.
(__glibc_unlikely, __glibc_likely): Use __builtin_expect also on older
clang versions.
(_Noreturn): Don't redefine on clang >= 3.5.
* lib/arg-nonnull.h (_GL_ARG_NONNULL): Use __attribute__ __nonnull__
also on clang.
* lib/dirent.in.h (_GL_ATTRIBUTE_PURE): Use __attribute__ __pure__ also
on clang.
* lib/stdlib.in.h (_GL_ATTRIBUTE_PURE): Likewise.
* lib/string.in.h (_GL_ATTRIBUTE_PURE): Likewise.
* lib/wchar.in.h (_GL_ATTRIBUTE_PURE): Likewise.
* lib/stdio.in.h (_GL_ATTRIBUTE_FORMAT): Use __attribute__ __format__
also on clang.
* lib/monetary.in.h (_GL_ATTRIBUTE_FORMAT): Likewise.
* lib/textstyle.in.h (ostream_printf, ostream_vprintf): Likewise.
* lib/unitypes.in.h (_UC_ATTRIBUTE_CONST): Use __attribute__ __const__
also on clang.
(_UC_ATTRIBUTE_PURE): Use __attribute__ __pure__ also on clang.
* lib/noreturn.h (_GL_NORETURN_FUNC, _GL_NORETURN_FUNCPTR): Use
__attribute__ __noreturn__ also on clang.
* lib/obstack.h (__attribute_noreturn__): Likewise.
* lib/file-set.h (record_file): Use __attribute__ __nonnull__ also on
clang.
* lib/argp-help.c (hol_entry_long_iterate): Use __attribute__
always_inline also on clang.
* tests/test-printf-posix.c (func1, func2, func3, func4): Test also on
clang.