Switch from a version 4.2/4.4 switch to a four-byte version string to be put
into the actual gcov file.

Instead of using the bottom 4 bytes as the function identifier, use a counter.
This makes the identifier numbers stable across multiple runs.
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.
Take the GCDA version string as an input to llvm_gcda_start_file.