EDG silently accepts deep pointer qualification conversions in C

[71iq]

The C front end reportedly accepts the following assignment without a
diagnostic:

void f(int **p)
{
  const int *const *q;
  q = p;
  (void)q;
}

This conversion is const-safe and valid in C++, but it violates the pointer
assignment constraints in C23 6.5.17.2 paragraph 1. The immediate pointed-to
types are not qualified or unqualified versions of compatible types. C23
5.1.1.3 paragraph 1 therefore requires a diagnostic; a warning is sufficient.

Standard reference: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3220.pdf

Suggested EDG reproduction: eccp --c --c23 -c testcase.c, also with
--strict_warnings. The EDG version and --dump_configuration output were
not supplied, and EDG execution is unavailable in this workspace. The silent
acceptance is the reported behavior, supported by inspecting current main
at 2d71da5.

GCC 13.3.0, with gcc -std=c2x -pedantic -fsyntax-only testcase.c, emits an
incompatible-pointer-types warning. Clang is also reported to diagnose this
conversion; it has not been independently rerun here.

In src/types.c, impl_pointer_conversion accepts the const-safe deep
qualification path as a C extension but forwards ec_no_error. A proposed
fix is to select the caller's default warning when C_mode() is true and
no more specific warning has been selected. It preserves extension
acceptance while providing the diagnostic. Related C++ work: PR #455.