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

[vvereschaka]

Summary

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 45526d29a5b2cf126b83ada3991921970007d16f.

[vvereschaka]

also we get the following cmake warning during the build

CMake Warning at C:/buildbot/as-builder-1/llvm-clang-win-x-armv7l/llvm-project/llvm/cmake/modules/AddLLVM.cmake:1638 (message):
  LLVM_EXTERNAL_LIT set to
  C:/buildbot/as-builder-1/llvm-clang-win-x-armv7l/build/./bin/llvm-lit, but
  the path does not exist.
Call Stack (most recent call first):
  C:/buildbot/as-builder-1/llvm-clang-win-x-armv7l/llvm-project/llvm/cmake/modules/AddLLVM.cmake:1682 (get_llvm_lit_path)
  C:/buildbot/as-builder-1/llvm-clang-win-x-armv7l/llvm-project/llvm/cmake/modules/AddLLVM.cmake:1727 (add_lit_target)
  C:/buildbot/as-builder-1/llvm-clang-win-x-armv7l/llvm-project/libunwind/test/CMakeLists.txt:33 (add_lit_testsuite)

(http://lab.llvm.org:8011/builders/llvm-clang-win-x-armv7l/builds/7142/steps/build-llvm-project/logs/stdio)

[stephenneuendorffer]

LGTM. thanks!

llvm/cmake/modules/CMakeLists.txt

set(LLVM_CONFIG_LIBRARY_DIRS
   "${LLVM_LIBRARY_DIR}"
   )
 set(LLVM_CONFIG_CMAKE_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
 set(LLVM_CONFIG_BINARY_DIR "${LLVM_BINARY_DIR}")
 set(LLVM_CONFIG_TOOLS_BINARY_DIR "${LLVM_TOOLS_BINARY_DIR}")
 
 # Generate a default location for lit
 if (LLVM_BUILD_UTILS)
+  if (CMAKE_HOST_WIN32 AND NOT CYGWIN)
+    set(LLVM_CONFIG_DEFAULT_EXTERNAL_LIT "${LLVM_CONFIG_TOOLS_BINARY_DIR}/llvm-lit.py")
+  else()
     set(LLVM_CONFIG_DEFAULT_EXTERNAL_LIT "${LLVM_CONFIG_TOOLS_BINARY_DIR}/llvm-lit")
   endif()
+endif()
 
 if (LLVM_LINK_LLVM_DYLIB)
   set(LLVM_CONFIG_LINK_LLVM_DYLIB
       "set(LLVM_LINK_LLVM_DYLIB ${LLVM_LINK_LLVM_DYLIB})")
 endif()
 
 # We need to use the full path to the LLVM Exports file to make sure we get the
 # one from the build tree. This is due to our cmake files being split between