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

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.

Differential Revision: https://reviews.llvm.org/D107700

(cherry picked from commit 614c7d0)

lldb/cmake/modules/LLDBStandalone.cmake

@@ -10,6 +10,8 @@ set(LLVM_MAIN_SRC_DIR ${LLVM_BUILD_MAIN_SRC_DIR} CACHE PATH "Path to LLVM source
 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")