PR tree-optimization/58689
* ansidecl.h (ATTRIBUTE_RETURNS_NONNULL): New macro.
* libiberty.h (basename, lbasename, dos_lbasename, unix_lbasename,
concat_copy): Mark with attributes nonnull(1) and returns_nonnull.
(concat_copy2, xstrerror): Mark with attribute returns_nonnull.This part is OK.
