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).
