[SanitizeCoverage] Enable stack-depth coverage for -fsanitize=fuzzer

Summary:
- Don't sanitize __sancov_lowest_stack.
- Don't instrument leaf functions.
- Add CoverageStackDepth to Fuzzer and FuzzerNoLink.

Reviewers: vitalybuka, kcc

Reviewed By: kcc

Subscribers: cfe-commits, llvm-commits, hiraditya

Differential Revision: https://reviews.llvm.org/D37156

llvm-svn: 311801