Tweak gdbsupport/valid-expr.h for GCC 6, fix build

Tweak gdbsupport/valid-expr.h for GCC 6, fix build

With GCC 6.4 and 6.5 (at least), unit tests that use
gdbsupport/valid-expr.h's CHECK_VALID fail to compile, with:

 In file included from src/gdb/unittests/offset-type-selftests.c:24:0:
 src/gdb/unittests/offset-type-selftests.c: In substitution of 'template<class Expected, template<class ...> class Op, class ... Args> using is_detected_exact = std::is_same<Expected, typename gdb::detection_detail::detector<gdb::nonesuch, void, Op, Args ...>::type> [with Expected = selftests::offset_type::off_A&; Op = selftests::offset_type::check_valid_expr75::archetype; Args = {selftests::offset_type::off_A, selftests::offset_type::off_B}]':
 src/gdb/unittests/offset-type-selftests.c:75:1:   required from here
 src/gdb/../gdbsupport/valid-expr.h:65:20: error: type/value mismatch at argument 2 in template parameter list for 'template<class Expected, template<class ...> class Op, class ... Args> using is_detected_exact = std::is_same<Expected, typename gdb::detection_detail::detector<gdb::nonesuch, void, Op, Args ...>::type>'
     archetype, TYPES>::value == VALID,   \
		     ^

The important part is the "error: type/value mismatch" error.  Seems
like that GCC doesn't understand that archetype is an alias template,
and is being strict in requiring a template class.

The fix here is then to make archetype a template class, to pacify
GCC.  The resulting code looks like this:

  template <TYPENAMES, typename = decltype (EXPR)>
  struct archetype
  {
  };

  static_assert (gdb::is_detected_exact<archetype<TYPES, EXPR_TYPE>,
 		 archetype, TYPES>::value == VALID, "");

is_detected_exact<Expected, Op, Args> checks whether Op<Args> is type
Expected:

 - For Expected, we pass the explicit EXPR_TYPE, overriding the
   default parameter type of archetype.

 - For Args we don't pass the last template parameter, so archtype
   defaults to the EXPR's decltype.

So in essence, we're really checking whether EXPR_TYPE is the same as
decltype(EXPR).

We need to do the decltype in a template context in order to trigger
SFINAE instead of failing to compile.

The hunk in unittests/enum-flags-selftests.c becomes necessary,
because unlike with the current alias template version, this new
version makes GCC trigger -Wenum-compare warnings as well:

 src/gdb/unittests/enum-flags-selftests.c:328:33: error: comparison between 'enum selftests::enum_flags_tests::RE' and 'enum selftests::enum_flags_tests::RE2' [-Werror=enum-compare]
  CHECK_VALID (true,  bool, RE () != RE2 ())
				  ^
 src/gdb/../gdbsupport/valid-expr.h:61:45: note: in definition of macro 'CHECK_VALID_EXPR_INT'
    template <TYPENAMES, typename = decltype (EXPR)>   \
					      ^

Build-tested with:

 - GCC {4.8.5, 6.4, 6.5, 7.3.1, 9.3.0, 11.0.0-20200910}
 - Clang 10.0.0

gdbsupport/ChangeLog:

	* valid-expr.h (CHECK_VALID_EXPR_INT): Make archetype a template
	class instead of an alias template and adjust static_assert.

gdb/ChangeLog:

	* unittests/enum-flags-selftests.c: Check whether __GNUC__ is
	defined before using '#pragma GCC diagnostic' instead of checking
	__clang__.