Add tests for coverage mapping generation.

This patch adds the tests for the coverage mapping generation. 
Most of the tests check the mapping regions produced by 
the generator, and one checks the llvm IR.

Differential Revision: http://reviews.llvm.org/D4793


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@215261 91177308-0d34-0410-b5e6-96231b3b80d8