[cmake] Silence a duplicate libraries warning from Apple's linker (#85012)

ld: warning: ignoring duplicate libraries:

This triggers quite frequently in llvm's build because CMake's library
depends mechanism doesn't de-duplicate libraries on the link line.
Duplication is necessary for ELF platforms, but means something subtly
different on Darwin platforms, hence the warning. Since we don't have
much control over that from CMake, just disable the warning wholesale
whenever the linker is detected to support it.

Author: jroelofs

llvm/cmake/modules/AddLLVM.cmake

@@ -257,6 +257,16 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
       message(STATUS "Linker detection: unknown")
     endif()
   endif()
+
+  # Apple's linker complains about duplicate libraries, which CMake likes to do
+  # to support ELF platforms. To silence that warning, we can use
+  # -no_warn_duplicate_libraries, but only in versions of the linker that
+  # support that flag.
+  if(NOT LLVM_USE_LINKER AND ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+    check_linker_flag(C "-Wl,-no_warn_duplicate_libraries" LLVM_LINKER_SUPPORTS_NO_WARN_DUPLICATE_LIBRARIES)
+  else()
+    set(LLVM_LINKER_SUPPORTS_NO_WARN_DUPLICATE_LIBRARIES OFF CACHE INTERNAL "")
+  endif()
 endif()
 
 function(add_link_opts target_name)
@@ -310,6 +320,11 @@ function(add_link_opts target_name)
     endif()
   endif()
 
+  if(LLVM_LINKER_SUPPORTS_NO_WARN_DUPLICATE_LIBRARIES)
+    set_property(TARGET ${target_name} APPEND_STRING PROPERTY
+                 LINK_FLAGS " -Wl,-no_warn_duplicate_libraries")
+  endif()
+
   if(ARG_SUPPORT_PLUGINS AND ${CMAKE_SYSTEM_NAME} MATCHES "AIX")
     set_property(TARGET ${target_name} APPEND_STRING PROPERTY
                  LINK_FLAGS " -Wl,-brtl")