[llvm] [cmake] Add check_linker_flag import (PR #85128)

[Justice Adams via llvm-commits]

https://github.com/justice-adams-apple created https://github.com/llvm/llvm-project/pull/85128

Fixing

```
CMake Error at cmake/llvm/AddLLVM.cmake:266 (check_linker_flag):
  Unknown CMake command "check_linker_flag".
```

>From 1c7d47530f4d36a339739135ffe255c3ce527b6e Mon Sep 17 00:00:00 2001
From: Justice Adams <107649528+justice-adams-apple at users.noreply.github.com>
Date: Wed, 13 Mar 2024 13:42:46 -0700
Subject: [PATCH] add check_linker_flag import

---
 llvm/cmake/modules/AddLLVM.cmake | 1 +
 1 file changed, 1 insertion(+)

diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index eb9e6101bdce71..f6fb56eb51e833 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -263,6 +263,7 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
   # -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")
+    include(CheckLinkerFlag)
     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 "")