Suggest if-let chain continuation on unclosed delimiter

[Diacod-I]

Meant to fix #160550. When an unclosed brace-delimited block begins with && or ||, the "unclosed delimiter" error now points at that line, suggesting the user may have meant to continue an if-let chain.

• Records the leading &&/|| span during lexing (in lex_token_tree_open_delim) and surfaces it in report_suspicious_mismatch_block.
• Adds a tests/ui/parser regression test.

• I did not use an LLM to create a change in this PR.
• I used an LLM to create a change in this PR, and I have explained below how it was used.

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

[rustbot]

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[estebank]

I seriously thought getting something working was going to be more complex than this ^_^'

We'd want to add some complexity later to better customize the message (there are some cases where this wasn't a let chain), but I'm ok with this for now.

@bors r+

[rust-bors]

📌 Commit 39b3ad8 has been approved by estebank

It is now in the queue for this repository.