[cmake] Add check_linker_flag import (llvm#85128)

Fixing

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

Author: justice-adams-apple

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 "")