[LLD] [COFF] Don't preserve unnecessary __imp_ prefixed symbols (#72989)

This redoes the fix from 3ab6209a
differently, without the unwanted effect of preserving unnecessary
`__imp_` prefixed symbols.

If the referencing object is a regular object, the `__imp_` symbol will
have `isUsedInRegularObj` set on it from that already. If the
referencing object is an LTO object, we set `isUsedInRegularObj` for any
symbol starting with `__imp_`.

If the object file defining the `__imp_` symbol is a regular object, the
`isUsedInRegularObj` flag has no effect. If it is an LTO object, it
causes the symbol to be preserved.