Update SanitizerCoverage doc regarding the issue with pc-table and gc-sections.
Update SanitizerCoverage doc regarding the issue with pc-table and gc-sections.

Summary:
There is a bug for this: https://bugs.llvm.org/show_bug.cgi?id=34636
But it would be also helpful to leave a note in the docs to prevent users from
running into issues, e.g. https://crbug.com/926588.

Reviewers: morehouse

Reviewed By: morehouse

Subscribers: cfe-commits, llvm-commits, kcc

Tags: #clang

Differential Revision: https://reviews.llvm.org/D57474

git-svn-id: ffe668792ed300d6c2daa1f6eba2e0aa28d7ec6c@352890 91177308-0d34-0410-b5e6-96231b3b80d8

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