Reapply "Rename/refactor isIntegerConstantExpression to getIntegerConstantExpression"

Reapply 49e5f603
which had been reverted in c9433291.

Originally reverted because I hadn't updated it in quite a while when I
got around to committing it, so there were a bunch of missing changes to
new code since I'd written the patch.

Reviewers: aaron.ballman

Differential Revision: https://reviews.llvm.org/D76646