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

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@312026 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,llvm-mirror/clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang,apple/swift-clang,apple/swift-clang,llvm-mirror/clang