Add IntegerAttr::verifyConstructionInvariants.

Summary:
This will help catch improper use of the MLIR API's. In particular, this
catches an error that was manifesting as nondeterministic assertion
failures (the nondeterminism was due to the failure happening only when the
StorageUniquer's DenseMap's probing happened to compare two specific
keys).

No test. The fact that all the existing tests pass with this additional
invariant gives confidence that it is correct/useful.

Differential Revision: https://reviews.llvm.org/D73645