[cmake] Add linker detection for Apple platforms

[smeenai]

Summary

LLVM currently assumes that Apple platforms will always use ld64. In the
future, LLD Mach-O might also be supported, so add the beginnings of
linker detection support. ld64 is currently the only detected linker,
since ld64.lld -v doesn't yield any useful version output, but we can
add that detection later, and in the meantime it's still useful to have
the ld64 identification.

Switch clang's order file check to use this new detection rather than
just checking for the presence of an ld64 executable.

[compnerd]

It really is unfortunate that linkers can spell version differently :-(. It would be really nice to be able to unify the linker version detection.

llvm/cmake/modules/AddLLVM.cmake

157

YES is better than ON here IMO

[smeenai]

llvm/cmake/modules/AddLLVM.cmake

157

I'll change this (and all the ones below) in a follow-up.

clang/tools/driver/CMakeLists.txt

if (APPLE)
 
   set(TOOL_INFO_UTI)
   set(TOOL_INFO_NAME)
   set(TOOL_INFO_VERSION)
   set(TOOL_INFO_BUILD_VERSION)
 endif()
 
 if(CLANG_ORDER_FILE AND
-   (LD64_EXECUTABLE OR LLVM_LINKER_IS_GOLD OR LLVM_LINKER_IS_LLD))
+    (LLVM_LINKER_IS_LD64 OR LLVM_LINKER_IS_GOLD OR LLVM_LINKER_IS_LLD))
   include(CheckLinkerFlag)
 
-  if (LD64_EXECUTABLE)
+  if (LLVM_LINKER_IS_LD64)
     set(LINKER_ORDER_FILE_OPTION "-Wl,-order_file,${CLANG_ORDER_FILE}")
   elseif (LLVM_LINKER_IS_GOLD)
     set(LINKER_ORDER_FILE_OPTION "-Wl,--section-ordering-file,${CLANG_ORDER_FILE}")
   elseif (LLVM_LINKER_IS_LLD)
     set(LINKER_ORDER_FILE_OPTION "-Wl,--symbol-ordering-file,${CLANG_ORDER_FILE}")
   endif()
 
   # This is a test to ensure the actual order file works with the linker.

llvm/cmake/modules/AddLLVM.cmake

function(add_llvm_symbol_exports target_name export_file)
 
   add_dependencies(${target_name} ${target_name}_exports)
 
   # Add dependency to *_exports later -- CMake issue 14747
   list(APPEND LLVM_COMMON_DEPENDS ${target_name}_exports)
   set(LLVM_COMMON_DEPENDS ${LLVM_COMMON_DEPENDS} PARENT_SCOPE)
 endfunction(add_llvm_symbol_exports)
 
-if(NOT WIN32 AND NOT APPLE)
+if(APPLE)
+  execute_process(
+    COMMAND "${CMAKE_LINKER}" -v
+    ERROR_VARIABLE stderr
+    )
+  set(LLVM_LINKER_DETECTED ON)
+  if("${stderr}" MATCHES "PROJECT:ld64")
+    set(LLVM_LINKER_IS_LD64 ON)
+    message(STATUS "Linker detection: ld64")
+  else()
+    set(LLVM_LINKER_DETECTED OFF)
+    message(STATUS "Linker detection: unknown")
+  endif()
+elseif(NOT WIN32)
   # Detect what linker we have here
   if( LLVM_USE_LINKER )
     set(command ${CMAKE_C_COMPILER} -fuse-ld=${LLVM_USE_LINKER} -Wl,--version)
   else()
     separate_arguments(flags UNIX_COMMAND "${CMAKE_EXE_LINKER_FLAGS}")
     set(command ${CMAKE_C_COMPILER} ${flags} -Wl,--version)
   endif()
   execute_process(