gdbsupport: ignore -Wenum-constexpr-conversion in enum-flags.h

gdbsupport: ignore -Wenum-constexpr-conversion in enum-flags.h

When building with clang 16, we get:

      CXX    gdb.o
    In file included from /home/smarchi/src/binutils-gdb/gdb/gdb.c:19:
    In file included from /home/smarchi/src/binutils-gdb/gdb/defs.h:65:
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/enum-flags.h:95:52: error: integer value -1 is outside the valid range of values [0, 15] for this enumeration type [-Wenum-constexpr-conversion]
        integer_for_size<sizeof (T), static_cast<bool>(T (-1) < T (0))>::type
                                                       ^

The error message does not make it clear in the context of which enum
flag this fails (i.e. what is T in this context), but it doesn't really
matter, we have similar warning/errors for many of them, if we let the
build go through.

clang is right that the value -1 is invalid for the enum type we cast -1
to.  However, we do need this expression in order to select an integer
type with the appropriate signedness.  That is, with the same signedness
as the underlying type of the enum.

I first wondered if that was really needed, if we couldn't use
std::underlying_type for that.  It turns out that the comment just above
says:

    /* Note that std::underlying_type<enum_type> is not what we want here,
       since that returns unsigned int even when the enum decays to signed
       int.  */

I was surprised, because std::is_signed<std::underlying_type<enum_type>>
returns the right thing.  So I tried replacing all this with
std::underlying_type, see if that would work.  Doing so causes some
build failures in unittests/enum-flags-selftests.c:

      CXX    unittests/enum-flags-selftests.o
    /home/smarchi/src/binutils-gdb/gdb/unittests/enum-flags-selftests.c:254:1: error: static assertion failed due to requirement 'gdb::is_same<selftests::enum_flags_tests::check_valid_expr254::archetype<enum_flags<s
    elftests::enum_flags_tests::RE>, selftests::enum_flags_tests::RE, enum_flags<selftests::enum_flags_tests::RE2>, selftests::enum_flags_tests::RE2, enum_flags<selftests::enum_flags_tests::URE>, selftests::enum_fla
    gs_tests::URE, int>, selftests::enum_flags_tests::check_valid_expr254::archetype<enum_flags<selftests::enum_flags_tests::RE>, selftests::enum_flags_tests::RE, enum_flags<selftests::enum_flags_tests::RE2>, selfte
    sts::enum_flags_tests::RE2, enum_flags<selftests::enum_flags_tests::URE>, selftests::enum_flags_tests::URE, unsigned int>>::value == true':
    CHECK_VALID (true,  int,  true ? EF () : EF2 ())
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/unittests/enum-flags-selftests.c:91:3: note: expanded from macro 'CHECK_VALID'
      CHECK_VALID_EXPR_6 (EF, RE, EF2, RE2, UEF, URE, VALID, EXPR_TYPE, EXPR)
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/valid-expr.h:105:3: note: expanded from macro 'CHECK_VALID_EXPR_6'
      CHECK_VALID_EXPR_INT (ESC_PARENS (typename T1, typename T2,           \
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    /home/smarchi/src/binutils-gdb/gdb/../gdbsupport/valid-expr.h:66:3: note: expanded from macro 'CHECK_VALID_EXPR_INT'
      static_assert (gdb::is_detected_exact<archetype<TYPES, EXPR_TYPE>,    \
      ^              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This is a bit hard to decode, but basically enumerations have the
following funny property that they decay into a signed int, even if
their implicit underlying type is unsigned.  This code:

    enum A {};
    enum B {};

    int main() {
      std::cout << std::is_signed<std::underlying_type<A>::type>::value
                << std::endl;
      std::cout << std::is_signed<std::underlying_type<B>::type>::value
                << std::endl;
      auto result = true ? A() : B();
      std::cout << std::is_signed<decltype(result)>::value << std::endl;
    }

produces:

    0
    0
    1

So, the "CHECK_VALID" above checks that this property works for enum flags the
same way as it would if you were using their underlying enum types.  And
somehow, changing integer_for_size to use std::underlying_type breaks that.

Since the current code does what we want, and I don't see any way of doing it
differently, ignore -Wenum-constexpr-conversion around it.

Change-Id: Ibc82ae7bbdb812102ae3f1dd099fc859dc6f3cc2