fix unused_braces suggestion for wildcard assignment

[pbkx]

Fixes #116536.

The unused_braces lint could suggest removing braces from wildcard assignments like:

_ = { e };

producing:

_ = e;

These are not equivalent: _ = { e }; moves/drops e, while _ = e; does not. This PR suppresses unused_braces for assignment RHS blocks when the assignment LHS contains _.

ui regression test covers both the simple wildcard assignment case and a tuple destructuring assignment case

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