[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