c++: Implement P2324R2, labels at the end of compound-stmts [PR103539]

c++: Implement P2324R2, labels at the end of compound-stmts [PR103539]

This patch implements C++23 <https://wg21.link/p2324r2>, which allows
labels at the end of a compound statement.   Its C FE counterpart was
already implemented in r11-4813.

In cp_parser_statement I rely on in_compound to determine whether we're
in a compound-statement, so that the patch doesn't accidentally allow

  void fn(int c) {
    if (c)
  label:
  }

Strangely, in_compound was reset after seeing a label (this is tested in
c-c++-common/gomp/pr63326.c), so I've made a modifiable copy specific
for OpenMP #pragma purposes.

	PR c++/103539

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_statement): Constify the in_compound parameter.
	Create a modifiable copy.  Allow labels at the end of compound
	statements.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp23/label1.C: New test.
	* g++.dg/cpp23/label2.C: New test.