Store ConstantExpr into the semantic attribute rather than calling getIntegerConstantExpr() for attribute dimension values

[smanna12]

We should be sure to double-check the definition of getXDimVal() and friends so that it uses the ConstantExpr stored into the semantic attribute rather than calling getIntegerConstantExpr(). Nothing to be done during this patch (if there's work here, it is better as a follow-up fix, I think).

Originally posted by @AaronBallman in #3170 (comment)