[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.
• Disable stack depth tracking on Mac.

Reviewers: vitalybuka, kcc, george.karpenkov

Reviewed By: kcc

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

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

llvm-svn: 312026