rustc: Start "stabilizing" some flags

This commit shuffles around some CLI flags of the compiler to some more stable
locations with some renamings. The changes made were:

* The `-v` flag has been repurposes as the "verbose" flag. The version flag has
  been renamed to `-V`.
* The `-h` screen has been split into two parts. Most top-level options (not
  all) show with `-h`, and the remaining options (generally obscure) can be
  shown with `--help -v` which is a "verbose help screen"
* The `-V` flag (version flag now) has lost its argument as it is now requested
  with `rustc -vV` "verbose version".
* The `--emit` option has had its `ir` and `bc` variants renamed to `llvm-ir`
  and `llvm-bc` to emphasize that they are LLVM's IR/bytecode.
* The `--emit` option has grown a new variant, `dep-info`, which subsumes the
  `--dep-info` CLI argument. The `--dep-info` flag is now deprecated.
* The `--parse-only`, `--no-trans`, `--no-analysis`, and `--pretty` flags have
  moved behind the `-Z` family of flags.
* The `--debuginfo` and `--opt-level` flags were moved behind the top-level `-C`
  flag.
* The `--print-file-name` and `--print-crate-name` flags were moved behind one
  global `--print` flag which now accepts one of `crate-name`, `file-names`, or
  `sysroot`. This global `--print` flag is intended to serve as a mechanism for
  learning various metadata about the compiler itself.
* The top-level `--pretty` flag was moved to a number of `-Z` options.

No warnings are currently enabled to allow tools like Cargo to have time to
migrate to the new flags before spraying warnings to all users.

cc https://github.com/rust-lang/rust/issues/19051
hey @alexcrichton , why did you put in this code that, AFAICT, is the identity function on `Option<T>` ?