Refactor GCOV's six constructor arguments into a struct with a getter that
constructs default arguments. It can now take default arguments from
cl::opt'ions. Add a new -default-gcov-version=... option, and actually test it!

Sink the reverse-order of the version into GCOVProfiling, hiding it from our
users.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@177002 91177308-0d34-0410-b5e6-96231b3b80d8