[lldb][CMake] Workaround debugserver code-signing issue in generated Xcode project
Explicitly code-sign the LLDB.framework copy of debugserver in the build-tree. This is necessary, because the Xcode-specific logic in llvm_codesign  has the side-effect that Xcode code-signs after post-build steps (here: after copying debugserver over into the framework). The special case for Xcode was necessary to avoid double-signing errors in the past (see D55116 and D55816).
Reviewers: jingham, davide, JDevlieghere, teemperor
Reviewed By: JDevlieghere
Subscribers: beanz, mgorny, lldb-commits, #lldb
Differential Revision: https://reviews.llvm.org/D65566