[clang][ExprConst] Can't be past an invalid LValue designator (#84293)

For the test case in C, both `LV.getLValueOffset()` and
`Ctx.getTypeSizeInChars(Ty)` are zero, so we return `true` from
`isOnePastTheEndOfCompleteObject()` and ultimately diagnose this as
being one past the end, but the diagnostic doesn't make sense.