[llvm-cov] - Included footer "Generated by llvm-cov -- llvm version <version number...

[llvm-cov] - Included footer "Generated by llvm-cov -- llvm version <version number>" in the coverage report.

The llvm-cov version information will be useful to the user when comparing the code coverage across different versions of llvm-cov. This patch provides the llvm-cov version information in the generated coverage report.

Differential Revision: https://reviews.llvm.org/D24457

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281321 91177308-0d34-0410-b5e6-96231b3b80d8