Added test for Core/Range class.

Summary:
We can optimize and refactor some of the classes in RangeMap.h, but first
we should have some tests for all the data structures in there. This adds a first
batch of tests for the Range class itself.

There are some unexpected results happening when mixing invalid and valid ranges, so
I added some FIXME's for that in the tests.

Reviewers: vsk

Reviewed By: vsk

Subscribers: mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D50620

llvm-svn: 339611