[clang] [llvm] [sancov] Add -diff and -union options to compute set difference and union of sancov files (PR #171191)

[Manuel Carrasco via cfe-commits]

mgcarrasco wrote:

@zhaoqi5 Thanks! I've tried to tag the most recent people involved in this tool. Would you have another reviewer in mind? 

As a side note, the new changes are not modifying any existing functionality so they can't cause regressions. 

https://github.com/llvm/llvm-project/pull/171191