c++: DR 569, DR 1693: fun with semicolons [PR113760]

c++: DR 569, DR 1693: fun with semicolons [PR113760]

Prompted by c++/113760, I started looking into a bogus "extra ;"
warning in C++11.  It quickly turned out that if I want to fix
this for good, the fix will not be so small.

This patch touches on DR 569, an extra ; at namespace scope should
be allowed since C++11:

  struct S {
  };
  ; // pedwarn in C++98

It also touches on DR 1693, which allows superfluous semicolons in
class definitions since C++11:

  struct S {
    int a;
    ; // pedwarn in C++98
  };

Note that a single semicolon is valid after a member function definition:

  struct S {
    void foo () {}; // only warns with -Wextra-semi
  };

There's a new function maybe_warn_extra_semi to handle all of the above
in a single place.  So now they all get a fix-it hint.

-Wextra-semi turns on all "extra ;" diagnostics.  Currently, options
like -Wc++11-compat or -Wc++11-extensions are not considered.

	DR 1693
	PR c++/113760
	DR 569

gcc/c-family/ChangeLog:

	* c.opt (Wextra-semi): Initialize to -1.

gcc/cp/ChangeLog:

	* parser.cc (extra_semi_kind): New.
	(maybe_warn_extra_semi): New.
	(cp_parser_declaration): Call maybe_warn_extra_semi.
	(cp_parser_member_declaration): Likewise.

gcc/ChangeLog:

	* doc/invoke.texi: Update -Wextra-semi documentation.

gcc/testsuite/ChangeLog:

	* g++.dg/diagnostic/semicolon1.C: New test.
	* g++.dg/diagnostic/semicolon10.C: New test.
	* g++.dg/diagnostic/semicolon11.C: New test.
	* g++.dg/diagnostic/semicolon12.C: New test.
	* g++.dg/diagnostic/semicolon13.C: New test.
	* g++.dg/diagnostic/semicolon14.C: New test.
	* g++.dg/diagnostic/semicolon15.C: New test.
	* g++.dg/diagnostic/semicolon16.C: New test.
	* g++.dg/diagnostic/semicolon17.C: New test.
	* g++.dg/diagnostic/semicolon2.C: New test.
	* g++.dg/diagnostic/semicolon3.C: New test.
	* g++.dg/diagnostic/semicolon4.C: New test.
	* g++.dg/diagnostic/semicolon5.C: New test.
	* g++.dg/diagnostic/semicolon6.C: New test.
	* g++.dg/diagnostic/semicolon7.C: New test.
	* g++.dg/diagnostic/semicolon8.C: New test.
	* g++.dg/diagnostic/semicolon9.C: New test.