llvm-cov: Added file checksum to gcno and gcda files.

Instead of permanently outputting "MVLL" as the file checksum, clang
will create gcno and gcda checksums by hashing the destination block
numbers of every arc. This allows for llvm-cov to check if the two gcov
files are synchronized.

Regenerated the test files so they contain the checksum. Also added
negative test to ensure error when the checksums don't match.

llvm-svn: 195191