Avoid unused braces lint for macro generated arguments

[chenyukang]

Fixes #158747

This changes the unused_braces lint to avoid suggesting brace removal for macro arguments when those braces may affect macro_rules!.

But the fragment kind matters:

• $e:expr matches an expression. Both 1 and { 1 } are expressions, so removing braces can still leave the macro matching the same kind of input.
• $b:block only matches a block expression written with braces. { 1 } matches, but 1 does not.

So call_expr!({ 1 }) can usually be simplified to call_expr!(1), while call_block!({ 1 }) cannot.

It seems a more concrect report need a full macro_rules! matcher analysis, that seems too heavy for unused lint.

[rustbot]

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