[llvm] Disable xar on macOS 12.0 and later

[JDevlieghere]

Summary

The xar file format has been deprecated since macOS 12.0, causing a warning to be printed:

warning: 'xar_open' is deprecated: first deprecated in macOS 12.0 - xar is a deprecated file format and should not be used.

Disable xar support when the macosx deployment target is greater or equal to 12.0.

[phosek]

LGTM

llvm/cmake/config-ix.cmake

218

Could we also print a warning that this option is deprecated and might be removed in future versions?

[JDevlieghere]

Print a status message about the deprecation of the xar file format and corresponding cmake variable.

llvm/cmake/config-ix.cmake

218

Is this what you had in mind? I didn't use an actual warning because the variable is set based on whether the library exists and therefore isn't really actionable. This seems like a nice middle ground.

[phosek]

llvm/cmake/config-ix.cmake

216–222

218

Yes, thanks.

[jyknight]

Isn't libxar still required by the lld.ld64 -bitcode_bundle option, so doesn't LLVM need to continue to use it despite it being deprecated?

llvm/cmake/config-ix.cmake

else()
     set(LLVM_ENABLE_TERMINFO 0)
   endif()
 else()
   set(LLVM_ENABLE_TERMINFO 0)
 endif()
 
 check_library_exists(xar xar_open "" LLVM_HAVE_LIBXAR)
 if(LLVM_HAVE_LIBXAR)
+  message(STATUS "The xar file format has been deprecated: LLVM_HAVE_LIBXAR might be removed in the future.")
+  # The xar file format has been deprecated since macOS 12.0.
+  if (CMAKE_OSX_DEPLOYMENT_TARGET VERSION_GREATER_EQUAL 12)
+    set(LLVM_HAVE_LIBXAR 0)
+  else()
     set(XAR_LIB xar)
   endif()
+endif()
 
 # function checks
 check_symbol_exists(arc4random "stdlib.h" HAVE_DECL_ARC4RANDOM)
 find_package(Backtrace)
 set(HAVE_BACKTRACE ${Backtrace_FOUND})
 set(BACKTRACE_HEADER ${Backtrace_HEADER})
 
 # Prevent check_symbol_exists from using API that is not supported for a given