rustc: Use static strings in a few literals

Avoid allocating extra copies of strings by using "" instead of ~"" for
the debug options list and for the `time` function. This is a small
change, but it is in a path that's always executed.
auto merge of #9717 : blake2-ppc/rust/rustc-static-str, r=alexcrichton

rustc: Use static strings in a few literals

Avoid allocating extra copies of strings by using "" instead of ~"" for
the debug options list and for the `time` function. This is a small
change, but it is in a path that's always executed.