Making the output of clang plugin more succinct

[aqjune]

Currently, validating gzip.c using clang-plugin emits a log which is 736 MB even if -tv-quiet is given.
Most of the output is either ERROR: Unsupported... or Transformation seems to be correct! (syntactically equal); removing all of them reduces the log size to 4MB.

It would be great if there is an option that makes the clang plugin even quieter.
I think printing only results that are made after SMT solver is finally invoked will significantly reduce the size of the log, but it may miss critical type mismatch errors in Alive2.