cmake: Use system llvm-lit when lit.py does not exist in srcdir

[mgorny]

Summary

Modify the add_lit_target function to use lit.py from LLVM_MAIN_SRC_DIR only when one does exist there, and otherwise fall back to looking for system install of llvm-lit. This is based on a similar conditional in clang's CMakeLists.txt, and makes it possible to run clang's tests when built separately from LLVM with no access to the original sources.

[mgorny]

Merged into D25076.

cmake/modules/AddLLVM.cmake

 # management functions.
 function(add_lit_target target comment)
   cmake_parse_arguments(ARG "" "" "PARAMS;DEPENDS;ARGS" ${ARGN})
   set(LIT_ARGS "${ARG_ARGS} ${LLVM_LIT_ARGS}")
   separate_arguments(LIT_ARGS)
   if (NOT CMAKE_CFG_INTDIR STREQUAL ".")
     list(APPEND LIT_ARGS --param build_mode=${CMAKE_CFG_INTDIR})
   endif ()
-  if (LLVM_MAIN_SRC_DIR)
+  if(EXISTS ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
     set (LIT_COMMAND ${PYTHON_EXECUTABLE} ${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py)
   else()
     find_program(LIT_COMMAND llvm-lit)
   endif ()
   list(APPEND LIT_COMMAND ${LIT_ARGS})
   foreach(param ${ARG_PARAMS})
     list(APPEND LIT_COMMAND --param ${param})
   endforeach()