Improve -Z unstable-options diagnostics and avoid an ICE

 * 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

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