[CMake] Detect properly new linker introduced in Xcode 15 (llvm#77806)

As explained in [1], this linker is functionally equivalent to the
classic one (`ld64`) for build system purposes -- in particular to 
enable the use of order files to link `clang`. For this reason, in 
addition to fixing the detection rename `LLVM_LINKER_IS_LD64` to 
`LLVM_LINKER_IS_APPLE` to make the result of such detection more 
clear -- this should not cause any issue to downstream users, from 
a quick search in SourceGraph [2], only Swift uses the value of
this variable (which I will take care of updating in due time).

[1]: https://developer.apple.com/documentation/xcode-release-notes/xcode-15-release-notes#Linking
[2]: https://sourcegraph.com/search?q=context:global+LLVM_LINKER_IS_LD64+lang:cmake+fork:no+-file:AddLLVM.cmake+-file:clang/tools/driver/CMakeLists.txt&patternType=standard&sm=1&groupBy=repo
rdar://120740222

Author: edymtt

clang/tools/driver/CMakeLists.txt

@@ -103,10 +103,10 @@ if (APPLE)
 endif()
 
 if(CLANG_ORDER_FILE AND
-    (LLVM_LINKER_IS_LD64 OR LLVM_LINKER_IS_GOLD OR LLVM_LINKER_IS_LLD))
+    (LLVM_LINKER_IS_APPLE OR LLVM_LINKER_IS_GOLD OR LLVM_LINKER_IS_LLD))
   include(LLVMCheckLinkerFlag)
 
-  if (LLVM_LINKER_IS_LD64 OR (LLVM_LINKER_IS_LLD AND APPLE))
+  if (LLVM_LINKER_IS_APPLE 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}")

llvm/cmake/modules/AddLLVM.cmake

@@ -211,10 +211,10 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
     )
 
   if(APPLE)
-    if("${stderr}" MATCHES "PROJECT:ld64")
+    if("${stderr}" MATCHES "PROGRAM:ld")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")
-      set(LLVM_LINKER_IS_LD64 YES CACHE INTERNAL "")
-      message(STATUS "Linker detection: ld64")
+      set(LLVM_LINKER_IS_APPLE YES CACHE INTERNAL "")
+      message(STATUS "Linker detection: Apple")
     elseif("${stderr}" MATCHES "^LLD" OR
            "${stdout}" MATCHES "^LLD")
       set(LLVM_LINKER_DETECTED YES CACHE INTERNAL "")