c++: Add testcases from some Issaquah DRs

c++: Add testcases from some Issaquah DRs

The following patch adds testcases for 5 DRs.  In the DR2475, DR2530 and
CWG2691 my understanding is we already implement the desired behavior,
in DR2478 partially (I've added 2 dg-bogus there, I think we inherit
rather than overwrite DECL_DECLARED_CONSTINIT_P for explicit specialization
somewhere, still far better than clang++) and DR2673 on the other side the
DR was to codify the clang++ behavior rather than GCC.

Not 100% sure if it is better to commit the 2 with dg-bogus or just wait
until the actual fixes are implemented.  BTW, I've noticed
register_specialization does:
              FOR_EACH_CLONE (clone, fn)
                {
                  DECL_DECLARED_INLINE_P (clone)
                    = DECL_DECLARED_INLINE_P (fn);
                  DECL_SOURCE_LOCATION (clone)
                    = DECL_SOURCE_LOCATION (fn);
                  DECL_DELETED_FN (clone)
                    = DECL_DELETED_FN (fn);
                }
but not e.g. constexpr/consteval, have tried to cover that in a testcase
but haven't managed to do so.

2023-02-15  Jakub Jelinek  <jakub@redhat.com>

	* g++.dg/DRs/dr2475.C: New test.
	* g++.dg/DRs/dr2478.C: New test.
	* g++.dg/DRs/dr2530.C: New test.
	* g++.dg/DRs/dr2673.C: New test.
	* c-c++-common/cpp/delimited-escape-seq-8.c: New test.