[Flang] Support overriding LLVM_LIT_ARGS in standalone builds (llvm#127340)

Declare `LLVM_LIT_ARGS` as a cache variable in standalone builds to
permit overriding it. This mirrors the logic used in Clang.

Author: mgorny

flang/CMakeLists.txt

@@ -199,6 +199,11 @@ if (FLANG_STANDALONE_BUILD)
     ${CMAKE_BINARY_DIR}/lib${LLVM_LIBDIR_SUFFIX})
 
   set(LLVM_EXTERNAL_LIT "${LLVM_TOOLS_BINARY_DIR}/llvm-lit" CACHE STRING "Command used to spawn lit")
+  set(LIT_ARGS_DEFAULT "-sv")
+  if (MSVC OR XCODE)
+    set(LIT_ARGS_DEFAULT "${LIT_ARGS_DEFAULT} --no-progress-bar")
+  endif()
+  set(LLVM_LIT_ARGS "${LIT_ARGS_DEFAULT}" CACHE STRING "Default options for lit")
 
   option(FLANG_INCLUDE_TESTS
          "Generate build targets for the Flang unit tests."