[SanitizerCoverage] Add stack depth tracing instrumentation.

Augment SanitizerCoverage to insert maximum stack depth tracing for
use by libFuzzer.  The new instrumentation is enabled by the flag
-fsanitize-coverage=stack-depth and is compatible with the existing
trace-pc-guard coverage.  The user must also declare the following
global variable in their code:
  thread_local uintptr_t __sancov_lowest_stack


Reviewers: vitalybuka, kcc

Reviewed By: vitalybuka

Subscribers: kubamracek, hiraditya, cfe-commits, llvm-commits

Differential Revision: https://reviews.llvm.org/D36839

git-svn-id: svn://svn.chromium.org/llvm-project/llvm/trunk/lib/Fuzzer@311186 0b72dbe1-c17f-4bc7-b9db-2b4152be0356
2 files changed