Add flags for additional control over coverage generation. Pick the version
string to be emitted, and two properties about the files themselves.

Use $PWD to absolut-ify the path to the coverage file. Yes, this is what GCC
does. Reverts my own r175706.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@176617 91177308-0d34-0410-b5e6-96231b3b80d8