Improve -Z unstable-options diagnostics and avoid an ICE

[rprichard]

• Consumers of handle_options assume the unstable options are defined in
  the getopts::Matches value if -Z unstable-options is set, but that's not
  the case if there weren't any actual unstable options. Fix this by
  always reparsing options when -Z unstable-options is set.

• If both argument parsing attempts fail, print the error from the second
  attempt rather than the first. The error from the first is very poor
  whenever unstable options are present. e.g.:

  $ rustc hello.rs -Z unstable-options --show-span
  error: Unrecognized option: 'show-span'.
  $ rustc hello.rs -Z unstable-options --pretty --pretty
  error: Unrecognized option: 'pretty'.
  $ rustc hello.rs -Z unstable-options --pretty --bad-option
  error: Unrecognized option: 'pretty'.

• On the second parse, add a separate pass to reject unstable options if
  -Z unstable-options wasn't specified.

Fixes #21715.
r? @pnkfelix

[pnkfelix]

@bors r+

[bors]

@bors r=pnkfelix 61004f8

[pnkfelix]

@bors rollup

[rprichard]

One thing I should mention: if the user attempts to use an unstable option without -Z unstable-options, and uses it incorrectly, they will get an error message regarding the misuse:

$ rustc hello.rs --show-span
error: Argument to option 'show-span' missing.

Ideally, I think they'd instead see this error:

$ rustc hello.rs --show-span=expr
error: use of unstable option 'show-span' requires -Z unstable-options

I don't see how to fix this, though, and the new behavior seemed better to me.