Split DIAG_* macros to new header libc-diag.h.

Split DIAG_* macros to new header libc-diag.h.

Quite a few tests include libc-internal.h just for the DIAG_* macros.
Split those macros to their own file, which can be included safely in
_ISOMAC mode.  I also moved ignore_value, since it seems logically
related, even though I didn't notice any tests needing it.

Also add -Wnonnull suppressions to two tests that _should_ have them,
but the error is masked when compiling against internal headers.

	* include/libc-diag.h: New file.  Define ignore_value,
	DIAG_PUSH_NEEDS_COMMENT, DIAG_POP_NEEDS_COMMENT,
	DIAG_IGNORE_NEEDS_COMMENT, and DIAG_IGNORE_Os_NEEDS_COMMENT here.

	* include/libc-internal.h: Definitions of above macros moved from
	here.  Include libc-diag.h.  Add copyright notice.

	* malloc/tst-malloc.c, malloc/tst-memcheck.c, malloc/tst-realloc.c
	* misc/tst-error1.c, posix/tst-dir.c, stdio-common/bug21.c
	* stdio-common/scanf14.c, stdio-common/scanf4.c, stdio-common/scanf7.c
	* stdio-common/test-vfprintf.c, stdio-common/tst-printf.c
	* stdio-common/tst-printfsz.c, stdio-common/tst-sprintf.c
	* stdio-common/tst-unlockedio.c, stdio-common/tstdiomisc.c
	* stdlib/bug-getcontext.c, string/tester.c, string/tst-endian.c
	* time/tst-strptime2.c, wcsmbs/tst-wcstof.c:
	Include libc-diag.h instead of libc-internal.h.

	* stdlib/tst-environ.c: Include libc-diag.h.  Suppress -Wnonnull for
	call to unsetenv (NULL).
	* nptl/tst-mutex1.c: Include libc-diag.h.  Suppress -Wnonnull for
	call to pthread_mutexattr_destroy (NULL).