[Darwin] CMake warning when building sanitized libLTO on Darwin with system sanitizer library (#176976)
Due to a system security policy, libLTO built with `LLVM_USE_SANITIZER`
and a toolchain (i.e. Xcode) sanitizer library cannot be loaded into the
toolchain `ld`. This only affects Darwin.
This adds a warning when users try to do this, and suggests a workaround
(use just-built sanitizer libraries).
This affected the lldb-cmake-sanitized job:
https://github.com/llvm/llvm-zorg/commits/main/zorg/jenkins/jobs/jobs/lldb-cmake-sanitized
rdar://168502870