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

[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

llvm-svn: 281321