Rollup merge of #128865 - jieyouxu:unicurd, r=Urgau

Ensure let stmt compound assignment removal suggestion respect codepoint boundaries

Previously we would try to issue a suggestion for `let x <op>= 1`, i.e.
a compound assignment within a `let` binding, to remove the `<op>`. The
suggestion code unfortunately incorrectly assumed that the `<op>` is an
exactly-1-byte ASCII character, but this assumption is incorrect because
we also recover Unicode-confusables like `➖=` as `-=`. In this example,
the suggestion code used a `+ BytePos(1)` to calculate the span of the
`<op>` codepoint that looks like `-` but the mult-byte Unicode
look-alike would cause the suggested removal span to be inside a
multi-byte codepoint boundary, triggering a codepoint boundary
assertion.

The fix is to use `SourceMap::start_point(token_span)` which properly accounts for codepoint boundaries.

Fixes #128845.

cc #128790

r? ````@fmease````

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