rust: pin-init: internal: make `DiagCtxt` available inside parser

Currently, there is a split between `syn::Error` and pin-init's custom
`DiagCtxt`. Parsing has no access to `DiagCtxt` and therefore cannot report
diagnostics without failing the parse. Bridge the gap by making the
`DiagCtxt` available inside parsers using `thread_local!`.

`parse_macro_input!` returns error as token stream directly, so it is no
longer suitable when the parsing is moved inside `DiagCtxt` closure. Add a
`From<syn::Error> for ErrorGuaranteed` implementation so `?` can be used to
add a `syn::Error` into the current diagnostic context and obtain a
`ErrorGuaranteed`.

Clean up the handler of using `<-` inside tuple expression as a
demonstration of the usefulness of having `DiagCtxt` access inside `Parse`.

Reviewed-by: Benno Lossin <lossin@kernel.org>
Link: https://patch.msgid.link/20260923140618.1978181-2-gary@kernel.org
Signed-off-by: Gary Guo <gary@garyguo.net>

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