[CMake] Ensure correct extension for llvm-lit is used on Windows when LLVM_INSTALL_UTILS is enabled.

[dgg5503]

Summary

D77110 initially added support for setting LLVM_CONFIG_DEFAULT_EXTERNAL_LIT
to llvm-lit in the install directory if LLVM_INSTALL_UTILS is on.

D79144 ensured that, on Windows, llvm-lit.py is correctly set for
LLVM_CONFIG_DEFAULT_EXTERNAL_LIT within the context of the build area,
however, it did not account for the install area which is the latter set
directive for this same variable.

This patch ensures that LLVM_CONFIG_DEFAULT_EXTERNAL_LIT under the install
area uses llvm-lit.py under Windows since llvm-lit without the extension
is not created.

[vvereschaka]

Hi @dgg5503, thank you, LGTM, but the pre-build status is failed. I don't see that failing of those tests are related with these changes. Anyway, would you double check it just in case?

[dgg5503]

Hi @vvereschaka, thank you for the quick approval! I'm not entirely sure why the tests are so flaky especially since I was able to pass all LIT tests under Windows and Linux locally. I've re-ran the pre-merge build 5 times and it appears that the three configurations pass if you combine runs 1 and 3. Is this enough to safely merge or should I continue rebuilding until all configurations passes?

If it is acceptable to merge, may you please submit this commit on my behalf as I do not have commit access? Feel free to use the following information for commit authoring:
Name: Douglas Gliner
E-mail: [e-mail]

[vvereschaka]

@dgg5503, I did one more rebuild and this time the result completely different. In comparison with the previous build, where libcxx ci and windows tests have failed, this time all those tests have passed, but the debian target tests have failed. I suppose, because these changes touch the common cmake configuration file, it triggers all possible cases/targets for the builds and not all of these tests are work correctly in that time.
So, the changes are not related with these failed tests. I'll commit your changes.

llvm/cmake/modules/CMakeLists.txt

 list(REMOVE_DUPLICATES LLVM_CONFIG_LIBRARY_DIRS)
 
 set(LLVM_CONFIG_BINARY_DIR "\${LLVM_INSTALL_PREFIX}")
 extend_path(LLVM_CONFIG_CMAKE_DIR "\${LLVM_INSTALL_PREFIX}" "${LLVM_INSTALL_PACKAGE_DIR}")
 extend_path(LLVM_CONFIG_TOOLS_BINARY_DIR "\${LLVM_INSTALL_PREFIX}" "${LLVM_TOOLS_INSTALL_DIR}")
 
 # Generate a default location for lit
 if (LLVM_INSTALL_UTILS AND 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()
 
 get_config_exports_includes(LLVM LLVM_CONFIG_INCLUDE_EXPORTS)
 set(LLVM_CONFIG_EXPORTS "${LLVM_EXPORTS}")
 configure_file(
   LLVMConfig.cmake.in
   ${CMAKE_CURRENT_BINARY_DIR}/CMakeFiles/LLVMConfig.cmake
   @ONLY)