Add a default directory for -Zmir-dump-dir

The current behaviour of dumping in the current directory is rarely
desirable: a sensible default directory for dumping is much more
convenient.

Author: varkor

src/librustc/session/config.rs

@@ -1172,7 +1172,7 @@ options! {DebuggingOptions, DebuggingSetter, basic_debugging_options,
           "emit noalias metadata for mutable references"),
     dump_mir: Option<String> = (None, parse_opt_string, [UNTRACKED],
           "dump MIR state at various points in translation"),
-    dump_mir_dir: Option<String> = (None, parse_opt_string, [UNTRACKED],
+    dump_mir_dir: Option<String> = (Some(String::from("mir_dump")), parse_opt_string, [UNTRACKED],
           "the directory the MIR is dumped into"),
     dump_mir_graphviz: bool = (false, parse_bool, [UNTRACKED],
           "in addition to `.mir` files, create graphviz `.dot` files"),