Fix the span information created by TokenStream's FromStr impl

[mystor]

This patch makes 3 changes:

1. StringReader would not update its peek_span or span fields when
   override_span was set, so this was moved into a method which
   updates the internal state.
2. StringReader was changed to hold a DUMMY_SP instead of a None
   for override_span, this decreases the size of StringReader and
   set's us up for (3).
3. StringReader::mk_sp was changed to copy the override_span's
   SyntaxContext, even if override_span was DUMMY_SP.
4. proc_macro::TokenStream::FromStr was updated to set the
   override_span to be based on DUMMY_SP instead of call_site.

This means that the line/column information for all tokens in the parsed
string will be correct, while also using the call site's hygiene
information.

Fixes#48944

[rust-highfive]

r? @pnkfelix

(rust_highfive has picked a reviewer for you, use r? to override)

[mystor]

r? @jseyfried

(I think you're probably the person to review this?)

[shepmaster]

Ping from triage, @jseyfried !

[mystor]

There are still some tests which are failing which I haven't had the time to figure out, so it's probably ok if you don't get around to reviewing it for a bit.

[emilyalbini]

Ping from the triage team @mystor! Will you have time to finish this PR in the near future?

[bors]

☔ The latest upstream changes (presumably #49154) made this pull request unmergeable. Please resolve the merge conflicts.

[emilyalbini]

Thank you for this PR @mystor! Unfortunately we haven't heard from you in a while, so I'm going to close this to keep things tidy.

Don't worry though, we would be happy to get this merged! If you'll have time in the future to work on this again please reopen the PR, and we'll review it!