Replace FALLTHROUGH macro by C++17 [[fallthrough]]

This attribute is now part of the standard we target so we no longer
need compiler-specific hacks.

Also enables -Wimplicit-fallthrough for Clang now that we can properly
support it. It's already on by default for GCC's -Wextra.

Fixes new warnings raised by Clang's -Wimplicit-fallthrough.

core/packed_data_container.cpp

@@ -225,8 +225,8 @@ uint32_t PackedDataContainer::_pack(const Variant &p_data, Vector<uint8_t> &tmpd
 
 			string_cache[s] = tmpdata.size();
 
-			FALLTHROUGH;
-		};
+			[[fallthrough]];
+		}
 		case Variant::NIL:
 		case Variant::BOOL:
 		case Variant::INT: