[lldb][CMake] Workaround debugserver code-signing issue in generated Xcode project

Summary:
Explicitly code-sign the LLDB.framework copy of debugserver in the build-tree. This is necessary, because the Xcode-specific logic in `llvm_codesign` [1] 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).

[1] https://github.com/llvm/llvm-project/blob/36fb93982f0e/llvm/cmake/modules/AddLLVM.cmake#L1676

Reviewers: jingham, davide, JDevlieghere, teemperor

Reviewed By: JDevlieghere

Subscribers: beanz, mgorny, lldb-commits, #lldb

Tags: #lldb

Differential Revision: https://reviews.llvm.org/D65566

llvm-svn: 368151