rust: proc-macro2: enable `proc_macro_span` feature

The `Span::join` function is very important in generating useful
diagnostics. When invoking `syn::Spanned::span`, it attempts to return an
overall span that covers the whole expression. If the `join` function is
unavailable, however, only the span of the first token is generated.

This causes worse diagnostics quality for all macros, notably pin-init,
when compared to the pin-init's own diagnostics test suite with the feature
enabled.

For example, this diagnostic covers only part of the text where replacement
is suggested:

 error: expected nothing or `..Zeroable::init_zeroed()`.
      --> tests/ui/compile-fail/zeroable/invalid_spread.rs:15:11
       |
    15 |         ..MyZeroable::init_zeroed()
       |           ^^^^^^^^^^

In the upcoming pin-init self-reference series, spans of full types or
expressions are used quite heavily to make diagnostics not confusing.

Thus, enable the `proc_macro_span` feature. This does make use of the
unstable `Span::join` function, but this is for producing diagnostics only,
so we do not have a hard reliance on the feature. A very small tweak of
proc-macro2 vendored code is needed to make the code compile for 1.85.

Link: https://github.com/rust-lang/rust/issues/54725
Link: https://github.com/dtolnay/proc-macro2/pull/542
Acked-by: Miguel Ojeda <ojeda@kernel.org>
Link: https://patch.msgid.link/20260921115708.4009754-1-gary@kernel.org/
[ Updated example diagnostics, removed unused PathBuf import. - Gary ]
Signed-off-by: Gary Guo <gary@garyguo.net>

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