Use C++11 fallthrough attribute syntax when available and add a break

[rnk]

Summary

This silences the two -Wimplicit-fallthrough warnings clang finds in
ItaniumDemangle.h in libc++abi.

Clang does not have a GNU attribute spelling for this attribute, so this
is necessary.

I will commit the same change to the LLVM demangler soon.

[ldionne]

Would it make sense to add the GNU spelling to the attribute in Clang?

[rnk]

In D53985#1284196, @ldionne wrote:

Would it make sense to add the GNU spelling to the attribute in Clang?

I'll redirect that question to @rsmith and @aaron.ballman. I have a vague recollection that it was considered and rejected, but that decision may have predated GCC 7.

In any case, libc++ needs the [[clang::fallthrough]] spelling to support old versions of clang in C++11 mode.

Thanks!

[aaron.ballman]

In D53985#1284246, @rnk wrote:

In D53985#1284196, @ldionne wrote:

Would it make sense to add the GNU spelling to the attribute in Clang?

I'll redirect that question to @rsmith and @aaron.ballman. I have a vague recollection that it was considered and rejected, but that decision may have predated GCC 7.

I'm not opposed to it given that it's an attribute supported by GCC.

[rsmith]

In D53985#1284282, @aaron.ballman wrote:

In D53985#1284246, @rnk wrote:

In D53985#1284196, @ldionne wrote:

Would it make sense to add the GNU spelling to the attribute in Clang?

I'll redirect that question to @rsmith and @aaron.ballman. I have a vague recollection that it was considered and rejected, but that decision may have predated GCC 7.

I'm not opposed to it given that it's an attribute supported by GCC.

Likewise; I think we decided not to do this before because (a) GCC didn't have such an attribute, and (b) it wasn't clear that we wanted to support "attribute statements" for GNU attributes at all, or whether this should be an attribute appertaining to the label.

libcxx/trunk/include/__config

#  define _LIBCPP_DIAGNOSE_WARNING(...) \
      __attribute__((diagnose_if(__VA_ARGS__, "warning")))
 #  define _LIBCPP_DIAGNOSE_ERROR(...) \
      __attribute__((diagnose_if(__VA_ARGS__, "error")))
 #else
 #  define _LIBCPP_DIAGNOSE_WARNING(...)
 #  define _LIBCPP_DIAGNOSE_ERROR(...)
 #endif
 
-#if __has_attribute(fallthough) || _GNUC_VER >= 700
 // Use a function like macro to imply that it must be followed by a semicolon
+#if __cplusplus > 201402L && __has_cpp_attribute(fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[fallthrough]]
+#elif __has_cpp_attribute(clang::fallthrough)
+#  define _LIBCPP_FALLTHROUGH() [[clang::fallthrough]]
+#elif __has_attribute(fallthough) || _GNUC_VER >= 700
 #  define _LIBCPP_FALLTHROUGH() __attribute__((__fallthrough__))
 #else
 #  define _LIBCPP_FALLTHROUGH() ((void)0)
 #endif
 
 #if defined(_LIBCPP_ABI_MICROSOFT) && \
     (defined(_LIBCPP_COMPILER_MSVC) || __has_declspec_attribute(empty_bases))
 #  define _LIBCPP_DECLSPEC_EMPTY_BASES __declspec(empty_bases)

libcxxabi/trunk/src/demangle/ItaniumDemangle.h

if (SoFar->getKind() == Node::KSpecialSubstitution) {
     switch (SSK) {
     case SpecialSubKind::string:
     case SpecialSubKind::istream:
     case SpecialSubKind::ostream:
     case SpecialSubKind::iostream:
       SoFar = make<ExpandedSpecialSubstitution>(SSK);
       if (!SoFar)
         return nullptr;
+      break;
     default:
       break;
     }
   }
 
   if (consumeIf('C')) {
     bool IsInherited = consumeIf('I');
     if (look() != '1' && look() != '2' && look() != '3' && look() != '5')