[6.3] Fix worker thread name handling on Linux#936

KushalP wants to merge 4 commits intoswiftlang:release/6.3.1swiftlang/swift-corelibs-libdispatch:release/6.3.1from

KushalP:release/6.3-worker-thread-name-cherry-pickKushalP/swift-corelibs-libdispatch:release/6.3-worker-thread-name-cherry-pickCopy head branch name to clipboard