Use "%zd" format specifier for printing number of testcases executed.
Use "%zd" format specifier for printing number of testcases executed.

Summary:
This helps to avoid signed integer overflow after running a fast fuzz target for several hours, e.g.:

<...>
Done -1097903291 runs in 54001 second(s)



Reviewers: kcc

Reviewed By: kcc

Differential Revision: https://reviews.llvm.org/D29941

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@295112 91177308-0d34-0410-b5e6-96231b3b80d8

GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,llvm-mirror/llvm,apple/swift-llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,apple/swift-llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm,GPUOpen-Drivers/llvm,llvm-mirror/llvm,llvm-mirror/llvm