Rename/refactor isIntegerConstantExpression to getIntegerConstantExpression

There is a version that just tests (also called
isIntegerConstantExpression) & whereas this version is specifically used
when the value is of interest (a few call sites were actually refactored
to calling the test-only version) so let's make the API look more like
it.

Reviewers: aaron.ballman

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