Generate error reports when a fuzz target exits.

Summary:
Implements https://github.com/google/sanitizers/issues/835.

Flush stdout before exiting in test cases.

Since the atexit hook is used for exit reports, pending prints to
stdout can be lost if they aren't flushed before calling exit().

Expect tests to have non-zero exit code if exit() is called.

Reviewers: vitalybuka, kcc

Reviewed By: kcc

Subscribers: eraman, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D35602

llvm-svn: 308669