[mlir] Support overriding LLVM_LIT_ARGS in standalone builds

[mgorny]

Summary

Introduce LLVM_LIT_ARGS cache variable in standalone builds, to let
the caller override the options passed by LLVM to lit calls. Once
defined, this is automatically handled by LLVM's cmake logic.

[mgorny]

Thanks!

mlir/CMakeLists.txt

if(MLIR_STANDALONE_BUILD)
   set(UNITTEST_DIR ${LLVM_MAIN_SRC_DIR}/utils/unittest)
   if(EXISTS ${UNITTEST_DIR}/googletest/include/gtest/gtest.h)
     add_subdirectory(${UNITTEST_DIR} utils/unittest)
   endif()
 
   set(CMAKE_LIBRARY_OUTPUT_DIRECTORY
     "${CMAKE_CURRENT_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX}")
   set(CMAKE_RUNTIME_OUTPUT_DIRECTORY "${CMAKE_CURRENT_BINARY_DIR}/bin")
+
+  set(LLVM_LIT_ARGS "-sv" CACHE STRING "Default options for lit")
 endif()
 
 set(MLIR_TOOLS_INSTALL_DIR "${CMAKE_INSTALL_BINDIR}" CACHE PATH
     "Path for binary subdirectory (defaults to '${CMAKE_INSTALL_BINDIR}')")
 mark_as_advanced(MLIR_TOOLS_INSTALL_DIR)
 
 set(MLIR_MAIN_SRC_DIR     ${CMAKE_CURRENT_SOURCE_DIR}  )
 set(MLIR_MAIN_INCLUDE_DIR ${MLIR_MAIN_SRC_DIR}/include )