Switch from a version 4.2/4.4 switch to a four-byte version string to be put

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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@176616 91177308-0d34-0410-b5e6-96231b3b80d8