[CMake] Provide a proper default location for llvm-lit for out-of-tree users on Windows host.

llvm-lit gets '.py' extension on Windows host during its configuration.
We need to provide a correct name for llvm-lit including file extension
within LLVM_CONFIG_DEFAULT_EXTERNAL_LIT variable.

Update for commit 45526d29.

Differential Revision: https://reviews.llvm.org/D79144