Fix the span information created by TokenStream's FromStr impl #48949
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This patch makes 3 changes:
StringReaderwould not update itspeek_spanorspanfields whenoverride_spanwas set, so this was moved into a method whichupdates the internal state.
StringReaderwas changed to hold aDUMMY_SPinstead of aNonefor
override_span, this decreases the size ofStringReaderandset's us up for (3).
StringReader::mk_spwas changed to copy theoverride_span'sSyntaxContext, even ifoverride_spanwasDUMMY_SP.proc_macro::TokenStream::FromStrwas updated to set theoverride_spanto be based onDUMMY_SPinstead ofcall_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