[lldb] [cmake] Add LLVM_LIT_ARGS override support for standalone builds

[mgorny]

Summary

Add a LLVM_LIT_ARGS cached variable in order to make it possible
to override lit arguments when doing standalone builds. Without that,
the user variable is ignored and the default options are always used.

Based on a similar solution found in clang.

[mgorny]

@JDevlieghere, could you also look at this one? I'm not really sure if anybody maintains LLDBStandalone at this point.

[JDevlieghere]

In D107700#2935385, @mgorny wrote:

@JDevlieghere, could you also look at this one? I'm not really sure if anybody maintains LLDBStandalone at this point.

I think it's pretty much just me these days. I would really like to get rid of it, but it's used downstream by the Swift build.

[JDevlieghere]

A possible alternative would be to export LLVM_LIT_ARGS from the llvm build so that lldb inherits it. I'm pretty much on the fence: on the one hand this doesn't really seems like something that should be exported, but on the other hand it'd be nice to not have to duplicate the option and having to update the default if it changes in llvm.

LGTM in the sense that I have no strong opinion.

[mgorny]

Well, I actually find it useful to be able to override lit args per project. Maybe some future solution would be to have it declared as a cache variable from some LLVM CMake file.

lldb/cmake/modules/LLDBStandalone.cmake

 option(LLVM_INSTALL_TOOLCHAIN_ONLY "Only include toolchain files in the 'install' target." OFF)
 
 find_package(LLVM REQUIRED CONFIG HINTS ${LLVM_DIR} NO_CMAKE_FIND_ROOT_PATH)
 find_package(Clang REQUIRED CONFIG HINTS ${Clang_DIR} ${LLVM_DIR}/../clang NO_CMAKE_FIND_ROOT_PATH)
 
 # We set LLVM_CMAKE_PATH so that GetSVN.cmake is found correctly when building SVNVersion.inc
 set(LLVM_CMAKE_PATH ${LLVM_CMAKE_DIR} CACHE PATH "Path to LLVM CMake modules")
 
 set(LLVM_MAIN_SRC_DIR ${LLVM_BUILD_MAIN_SRC_DIR} CACHE PATH "Path to LLVM source tree")
 set(LLVM_MAIN_INCLUDE_DIR ${LLVM_MAIN_INCLUDE_DIR} CACHE PATH "Path to llvm/include")
 set(LLVM_BINARY_DIR ${LLVM_BINARY_DIR} CACHE PATH "Path to LLVM build tree")
 
+set(LLVM_LIT_ARGS "-sv" CACHE STRING "Default options for lit")
+
 set(lit_file_name "llvm-lit")
 if(CMAKE_HOST_WIN32 AND NOT CYGWIN)
   set(lit_file_name "${lit_file_name}.py")
 endif()
 
 function(append_configuration_directories input_dir output_dirs)
   set(dirs_list ${input_dir})
   foreach(config_type ${LLVM_CONFIGURATION_TYPES})