[Bug c++/97913] -fno-delete-null-pointer-checks not working properly with constexpr

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97913

--- Comment #6 from Alexey Klimkin <alexey.klimkin at intel dot com> ---
Created attachment 65252
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65252&action=edit
Proposed patch: keep constant evaluation independent of null-check optimization

I attached a proposed patch for PR71962 and PR97913. -fsanitize=null implicitly
disables -fdelete-null-pointer-checks, but that optimization setting should not
change which C++ expressions are accepted as constant expressions.

The patch makes maybe_nonzero_address answer as if null-pointer-check deletion
were enabled only during manifest constant evaluation. Runtime folding still
honors the option, and undefined weak symbols remain conservative. The tests
cover direct -fno-delete-null-pointer-checks, -fsanitize=null, runtime
weak-symbol comparisons, and an undefined weak symbol.

Focused before/after testing shows the constant-expression cases fail without
the patch and pass with it, while the two runtime weak-symbol comparisons
remain in optimized output. The patch is submitted under the DCO sign-off
included in the attachment.