Improve `-Z crate-attr` diagnostics

- Show the `#![ ... ]` in the span (to make it clear that it should not
  be included in the CLI argument)
- Show more detailed errors when the crate has valid token trees but
  invalid syntax.
  Previously, `crate-attr=feature(foo),feature(bar)` would just say
  "invalid crate attribute" and point at the comma. Now, it explicitly
  says that the comma was unexpected, which is useful when using
  `--error-format=short`. It also fixes the column to show the correct
  span.
- Recover from parse errors. Previously we would abort immediately on
  syntax errors; now we go on to try and type-check the rest of the
  crate.

The new diagnostic code also happens to be slightly shorter.

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