[CMake] Rename lldb-launcher to darwin-debug

Summary: This tool is only built on Darwin, and the name darwin-debug matches the Xcode project. We should have this in sync unless there is a good reason not to.

Reviewers: zturner, tfiala, labath

Subscribers: labath, mgorny, lldb-commits

Differential Revision: https://reviews.llvm.org/D25745

llvm-svn: 285356