[libFuzzer] Merge: print stats after reading the output corpus dir.

Summary:
The purpose is to be able to extract the number of new edges added to
the original (i.e. output) corpus directory after doing the merge. Use case
example: in ClusterFuzz, we do merge after every fuzzing session, to avoid
uploading too many corpus files, and we also record coverage stats at that
point. Having a separate line indicating stats after reading the initial output
corpus directory would make the stats extraction easier for both humans and
parsing scripts.

Context: https://github.com/google/clusterfuzz/issues/802.

Reviewers: morehouse, hctim

Reviewed By: hctim

Subscribers: delcypher, #sanitizers, llvm-commits, kcc

Tags: #llvm, #sanitizers

Differential Revision: https://reviews.llvm.org/D66020

llvm-svn: 368461