InstrProf: Make CoverageMapping testable and add a basic unit test

Make CoverageMapping easier to create, so that we can write targeted
unit tests for its internals, and add a some infrastructure to write
these tests. Finally, add a simple unit test for basic functionality.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229709 91177308-0d34-0410-b5e6-96231b3b80d8