Remove is_integer_const

[Jarcho]

Most instances of is_integer_const were used by style lints which never should have used it in the first place. e.g. range_plus_one will no longer suggest changing x..y + C to x..=y just because C is currently defined as one; it's a thing which can change. The final uses of it have been replaced with eval_int which gives a notably better API in one case a mildly worse one in the other case.

changelog: none

[rustbot]

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