New lint: collapsible_tuple_let

[Souradip121]

Adds the collapsible_tuple_let lint (style, warn-by-default).

Detects let (a, b) = { let x = expr; (x, expr2) }; where the block
is unnecessary — the expressions are fully separable and can be written
as individual let statements.

Closes #16750

What the lint catches

// Before
let (var1, var2) = {
    let v1 = a_complex_expression(&mut inputs)?;
    (v1, another_complex_expression(inputs))
};

// After (MachineApplicable suggestion)
let var1 = a_complex_expression(&mut inputs)?;
let var2 = another_complex_expression(inputs);

Conservative conditions (no false positives)

The lint is skipped when:

• Block-locals appear in reversed / non-declaration order in the tuple
  (would reorder side effects)
• A block-local is used in another block statement's initializer
• An inline expression precedes a block-local reference in the tuple
• The same block-local is referenced twice in the tuple
• The outer let carries a type annotation or let-else
• Any relevant span comes from a macro expansion

changelog: [collapsible_tuple_let]: Warn when a let tuple destructuring can be collapsed into individual let statements by removing an unnecessary block expression.

[rustbot]

r? @llogiq

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Why was this reviewer chosen?

The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
• 7 candidates expanded to 7 candidates
• Random selection from Jarcho, dswij, llogiq, samueltardieu

[github-actions]

Lintcheck changes for 9955051

Lint	Added	Removed	Changed
clippy::collapsible_tuple_let	1	0	0

This comment will be updated if you push new changes

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.