[CMake] Allow devs to disable implicit incremental testing

Lit's incremental mode is incompatible with read-only source filesystems
which some CI setups use and there is no way to disable incremental mode
in Swift without enabling --shuffle via LLVM_LIT_ARGS.

Author: davezarzycki

test/CMakeLists.txt

@@ -120,26 +120,26 @@ set(LIT "${LLVM_MAIN_SRC_DIR}/utils/lit/lit.py")
 # instead of by the path, which is good for CI. In this mode lit also updates
 # the mtime on the failed tests, which makes them run first on the
 # consecutive execution, which makes local builds fail faster.
-set(SWIFT_TEST_EXTRA_ARGS "--incremental")
+set(SWIFT_LIT_ARGS "--incremental" CACHE STRING "Arguments to pass to lit")
 
 if(NOT SWIFT_INCLUDE_TOOLS)
-  list(APPEND SWIFT_TEST_EXTRA_ARGS
+  list(APPEND SWIFT_LIT_ARGS
        "--path=${SWIFT_NATIVE_LLVM_TOOLS_PATH}"
        "--path=${SWIFT_NATIVE_CLANG_TOOLS_PATH}"
        "--path=${SWIFT_NATIVE_SWIFT_TOOLS_PATH}")
   if(SWIFT_BUILD_STDLIB)
-    list(APPEND SWIFT_TEST_EXTRA_ARGS
+    list(APPEND SWIFT_LIT_ARGS
          "--param" "test_resource_dir=${SWIFTLIB_DIR}")
   endif()
 endif()
 
 option(SWIFT_TEST_USE_LEAKS "Run Swift stdlib tests under leaks" FALSE)
 if (SWIFT_TEST_USE_LEAKS)
-  list(APPEND SWIFT_TEST_EXTRA_ARGS "--param" "leaks-all")
+  list(APPEND SWIFT_LIT_ARGS "--param" "leaks-all")
 endif()
 
 if(NOT CMAKE_CFG_INTDIR STREQUAL ".")
-  list(APPEND SWIFT_TEST_EXTRA_ARGS
+  list(APPEND SWIFT_LIT_ARGS
        "--param" "build_mode=${CMAKE_CFG_INTDIR}")
 endif()
 
@@ -332,7 +332,7 @@ _Block_release(void) { }\n")
         COMMENT "Uploading stdlib")
 
     foreach(test_mode ${TEST_MODES})
-      set(LIT_ARGS "${SWIFT_TEST_EXTRA_ARGS} ${LLVM_LIT_ARGS}")
+      set(LIT_ARGS "${SWIFT_LIT_ARGS} ${LLVM_LIT_ARGS}")
       separate_arguments(LIT_ARGS)
 
       if(NOT SWIFT_BUILD_STDLIB AND NOT SWIFT_PATH_TO_EXTERNAL_STDLIB_BUILD)