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

[kastiglione]

Summary

When using a custom LLVM_EXTERNAL_LIT, it's possible the file may not exist at the time CMake is generating the build. One example is LLDB standalone builds. When the external lit doesn't exist, a warning message is emitted, but the warning is printed once for every lit target. This produces many redundant warnings.

This changes the warning to only be emitted once, controlled by a CACHE variable.

Other options are:

1. remove the warning
2. have callers pass an option to silence the warning if desired

See https://reviews.llvm.org/D76945 for some context.

[kastiglione]

else/if -> elseif

[JDevlieghere]

llvm/cmake/modules/AddLLVM.cmake

1661

This should probably be of type INTERNAL which is the same as CACHE but without the (G)UI entry.

[kastiglione]

Use type INTERNAL

llvm/cmake/modules/AddLLVM.cmake

1661

nice, done.

[JDevlieghere]

LGTM. Thanks for fixing this!

llvm/cmake/modules/AddLLVM.cmake

if (ARG_ALLOW_EXTERNAL)
 
     if (NOT "${LLVM_EXTERNAL_LIT}" STREQUAL "")
       if (EXISTS ${LLVM_EXTERNAL_LIT})
         get_filename_component(LIT_FILE_NAME ${LLVM_EXTERNAL_LIT} NAME)
         get_filename_component(LIT_BASE_DIR ${LLVM_EXTERNAL_LIT} DIRECTORY)
         set(${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 BOOL "")
       endif()
     endif()
   endif()
 
   set(lit_file_name "llvm-lit")
   if (CMAKE_HOST_WIN32 AND NOT CYGWIN)
     # llvm-lit needs suffix.py for multiprocess to find a main module.
     set(lit_file_name "${lit_file_name}.py")