I tried to compile LLD using GCC 7.1.0 and got warnings like
"warning: this statement may fall through [-Wimplicit-fallthrough=]"
(some more details are here: D32907)
GCC's __cplusplus value is 201402L by default, so macro expands to nothing,
though GCC 7 has support for [[fallthrough]].
I suggest to use gnu::fallthrough when it is available. That fixes issue I am observing.