[cmake] Make LLVM_ENABLE_LLD=ON work better on macOS

[thakis]

Summary

LLVM_LINKER_IS_LLD is now set with LLVM_ENABLE_LLD=ON (or LLVM_USER_LINKER=lld)
even on APPLE, and we pass -Wl,-order_file when LLVM_LINKER_IS_LLD on APPLE
too.

To make this straightforward, change the linker detection logic to go through
the compiler driver on APPLE like on the other platforms.

No intended behavior change if LLVM_ENABLE_LLD isn't set.

[hans]

lgtm with one question

llvm/cmake/modules/AddLLVM.cmake

170

nit: maybe add a period while here.

179

nit: maybe drop the spaces while here.

196

isn't it enough to check one stream?

[thakis]

llvm/cmake/modules/AddLLVM.cmake

196

Yes, but lld currently writes it to stdout – but since ld64 writes to stderr that's an lld bug that we should fix (in D113020 :) ) so then this needs to support both behaviors, at least for a while.

clang/tools/driver/CMakeLists.txt

if (APPLE)
   set(TOOL_INFO_VERSION)
   set(TOOL_INFO_BUILD_VERSION)
 endif()
 
 if(CLANG_ORDER_FILE AND
     (LLVM_LINKER_IS_LD64 OR LLVM_LINKER_IS_GOLD OR LLVM_LINKER_IS_LLD))
   include(LLVMCheckLinkerFlag)
 
-  if (LLVM_LINKER_IS_LD64)
+  if (LLVM_LINKER_IS_LD64 OR (LLVM_LINKER_IS_LLD and APPLE))
     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 DEFINED LLVM_LINKER_DETECTED)
+if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
+  # Detect what linker we have here.
   if(APPLE)
-    execute_process(
-      COMMAND "${CMAKE_LINKER}" -v
-      ERROR_VARIABLE stderr
-      )
+    # Linkers with ld64-compatible flags.
+    set(version_flag "-Wl,-v")
+  else()
+    # Linkers with BFD ld-compatible flags.
+    set(version_flag "-Wl,--version")
+  endif()
+
+  if(LLVM_USE_LINKER)
+    set(command ${CMAKE_C_COMPILER} -fuse-ld=${LLVM_USE_LINKER} ${version_flag})
+  else()
+    separate_arguments(flags UNIX_COMMAND "${CMAKE_EXE_LINKER_FLAGS}")
+    set(command ${CMAKE_C_COMPILER} ${flags} ${version_flag})
+  endif()
+    COMMAND ${command}
+    OUTPUT_VARIABLE stdout
+
+  if(APPLE)
     if("${stderr}" MATCHES "PROJECT:ld64")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
       set(LLVM_LINKER_IS_LD64 YES CACHE INTERNAL "")
       message(STATUS "Linker detection: ld64")
+    elseif("${stderr}" MATCHES "^LLD" OR
+           "${stdout}" MATCHES "^LLD")
+      set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
+      set(LLVM_LINKER_IS_LLD YES CACHE INTERNAL "")
+      message(STATUS "Linker detection: lld")
     else()
       set(LLVM_LINKER_DETECTED NO CACHE INTERNAL "")
       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(
-      COMMAND ${command}
-      OUTPUT_VARIABLE stdout
-      ERROR_VARIABLE stderr
-      )
     if("${stdout}" MATCHES "^mold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
       message(STATUS "Linker detection: mold")
     elseif("${stdout}" MATCHES "GNU gold")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
       set(LLVM_LINKER_IS_GOLD YES CACHE INTERNAL "")
       message(STATUS "Linker detection: GNU Gold")
     elseif("${stdout}" MATCHES "^LLD")