Merge pull request #1984 from apple/cmake-Limit-missing-external-lit-warning-to-be-shown-once

[cmake] Limit missing external lit warning to be shown once

Author: kastiglione

llvm/cmake/modules/AddLLVM.cmake

@@ -1669,8 +1669,9 @@ function(get_llvm_lit_path base_dir file_name)
         set(${file_name} ${LIT_FILE_NAME} PARENT_SCOPE)
         set(${base_dir} ${LIT_BASE_DIR} PARENT_SCOPE)
         return()
-      else()
+      elseif (NOT DEFINED CACHE{LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE})
         message(WARNING "LLVM_EXTERNAL_LIT set to ${LLVM_EXTERNAL_LIT}, but the path does not exist.")
+        set(LLVM_EXTERNAL_LIT_MISSING_WARNED_ONCE YES CACHE INTERNAL "")
       endif()
     endif()
   endif()