Auto merge of rust-lang#2311 - RalfJung:many-seeds, r=RalfJung

./miri many-seeds: also print the seed before we try it

When using `cargo miri`, we otherwise have no way of even seeing which seed it is currently on.

miri

@@ -55,6 +55,7 @@ COMMAND="$1"
 case "$COMMAND" in
 many-seeds)
     for SEED in $({ echo obase=16; seq 0 255; } | bc); do
+        echo "Trying seed: $SEED"
         MIRIFLAGS="$MIRIFLAGS -Zmiri-seed=$SEED" $@ || { echo "Failing seed: $SEED"; break; }
     done
     exit 0