Use __attribute__((fallthrough)) rather than magic GCC comments.

GCC added an implicit-fallthrough warning a while back, where it
would complain if you had a nontrivial "case:" block that didn't end
with break, return, or something like that.  Clang recently added
the same thing.

GCC, however, would let you annotate a fall-through as intended by
any of various magic "/* fall through */" comments.  Clang, however,
only seems to like "__attribute__((fallthrough))".  Fortunately, GCC
accepts that too.

A previous commit in this branch defined a FALLTHROUGH macro to do
the right thing if GNUC is defined; here we replace all of our "fall
through" comments with uses of that macro.

This is an automated commit, made with the following perl one-liner:

  #!/usr/bin/perl -i -p
  s#/\* *falls? ?thr.*?\*/#FALLTHROUGH;#i;

1 files changed, 1 insertions, 1 deletions

diff --git a/src/feature/relay/dns.c b/src/feature/relay/dns.c
index 38395862c3..e20a39482f 100644
--- a/src/feature/relay/dns.c
+++ b/src/feature/relay/dns.c
@@ -548,7 +548,7 @@ send_resolved_cell,(edge_connection_t *conn, uint8_t answer_type,
         answer_type = RESOLVED_TYPE_ERROR;
         /* We let this fall through and treat it as an error. */
       }
-      /* Falls through. */
+      FALLTHROUGH;
     case RESOLVED_TYPE_ERROR_TRANSIENT:
     case RESOLVED_TYPE_ERROR:
       {