[CMake] Fix linker detection in AddLLVM.cmake

Fix linker not being correctly detected when a custom one is specified
through LLVM_USE_LINKER CMake variable.

In particular,

  cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_LINKER=gold ../llvm

resulted into

  Linker detection: GNU ld

instead of

  Linker detection: GNU Gold

due to the construction not accounting for such variable. It led to the general
confusion and prevented setting linker-specific flags inside functions defined
in AddLLVM.cmake.

Thanks Oleksii Vilchanskyi for the patch!

llvm-svn: 316956
diff --git a/llvm/cmake/modules/AddLLVM.cmake b/llvm/cmake/modules/AddLLVM.cmake
index f52b1c4..62e264e 100644
--- a/llvm/cmake/modules/AddLLVM.cmake
+++ b/llvm/cmake/modules/AddLLVM.cmake
@@ -149,8 +149,13 @@
 
 if(NOT WIN32 AND NOT APPLE)
   # Detect what linker we have here
+  if( LLVM_USE_LINKER )
+    set(command ${CMAKE_C_COMPILER} -fuse-ld=${LLVM_USE_LINKER} -Wl,--version)
+  else()
+    set(command ${CMAKE_C_COMPILER} -Wl,--version)
+  endif()
   execute_process(
-    COMMAND ${CMAKE_C_COMPILER} -Wl,--version
+    COMMAND ${command}
     OUTPUT_VARIABLE stdout
     ERROR_VARIABLE stderr
     )