Suggestion for incorrect `repr` use

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When encountering #[repr = "C"], suggest #[repr(C)] instead of the current #[repr(C, packed, ...)]. We need to detect specific cases like writing #[repr("packed")], where we should suggest #[repr(packed)], as otherwise provide a list of all the valid reprs.

(Follow up to #61140)

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Current output:

rust/tests/ui/repr/repr.stderr

Lines 20 to 41 in f5711a5

	error[E0539]: malformed `repr` attribute input
	--> $DIR/repr.rs:4:1
	|
	LL | #[repr = "B"]
	| ^^^^^^^^^^^^^ expected this to be a list
	|
	= note: for more information, visit <https://doc.rust-lang.org/reference/type-layout.html#representations>
	help: try changing it to one of the following valid forms of the attribute
	|
	LL - #[repr = "B"]
	LL + #[repr(<integer type>)]
	|
	LL - #[repr = "B"]
	LL + #[repr(C)]
	|
	LL - #[repr = "B"]
	LL + #[repr(Rust)]
	|
	LL - #[repr = "B"]
	LL + #[repr(align(...))]
	|
	= and 2 other candidates