InstrProf: Add some unit tests for CoverageMapping

InstrProf: Add some unit tests for CoverageMapping

The llvm-level tests for coverage mapping need a binary input file,
which means they're hard to understand, hard to update, and it's
difficult to add new ones. By adding some unit tests that build up the
coverage data structures in C++, we can write more meaningful and
targeted tests.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@228084 91177308-0d34-0410-b5e6-96231b3b80d8