[cmake] Move HandleOutOfTreeLLVM to common cmake utils

[Ericson2314]

Summary

This is better than libunwind and libcxxabi fishing it out of libcxx's
module directory.

It is done in prepartion for a better version of D117537 which deduplicates
CMake logic instead of just renaming to avoid a name clash.

[phosek]

LGTM we should see if we can also use HandleOutOfTreeLLVM.cmake in compiler-rt in a follow-up change.

[Ericson2314]

Hmm not sure why it's not going to the accepted state. I guess I did something odd when I marked it needs more review before.

LGTM we should see if we can also use HandleOutOfTreeLLVM.cmake in compiler-rt in a follow-up change.

That is indeed what I ended up doing! :)

cmake/Modules/HandleOutOfTreeLLVM.cmake

 if (NOT DEFINED LLVM_PATH)
-  set(LLVM_PATH ${CMAKE_CURRENT_LIST_DIR}/../../../llvm CACHE PATH "" FORCE)
+  set(LLVM_PATH ${CMAKE_CURRENT_LIST_DIR}/../../llvm CACHE PATH "" FORCE)
 endif()
 
 if(NOT IS_DIRECTORY ${LLVM_PATH})
   message(FATAL_ERROR
     "The provided LLVM_PATH (${LLVM_PATH}) is not a valid directory. Note that "
     "building libc++ outside of the monorepo is not supported anymore. Please "
     "use a Standalone build against the monorepo, a Runtimes build or a classic "
     "monorepo build.")

libcxxabi/CMakeLists.txt

 if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBCXXABI_STANDALONE_BUILD)
   project(libcxxabi CXX C)
 
   set(PACKAGE_NAME libcxxabi)
   set(PACKAGE_VERSION 11.0.0git)
   set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
   set(PACKAGE_BUGREPORT "[e-mail]")
 
-  # Add the CMake module path of libcxx so we can reuse HandleOutOfTreeLLVM.cmake
-  set(LIBCXXABI_LIBCXX_CMAKE_PATH "${LIBCXXABI_LIBCXX_PATH}/cmake/Modules")
-  list(APPEND CMAKE_MODULE_PATH "${LIBCXXABI_LIBCXX_CMAKE_PATH}")
-
   # In a standalone build, we don't have llvm to automatically generate the
   # llvm-lit script for us.  So we need to provide an explicit directory that
   # the configurator should write the script into.
   set(LIBCXXABI_STANDALONE_BUILD 1)
   set(LLVM_LIT_OUTPUT_DIR "${LIBCXXABI_BINARY_DIR}/bin")
 
   # Find the LLVM sources and simulate LLVM CMake options.
   include(HandleOutOfTreeLLVM)

libunwind/CMakeLists.txt

if (CMAKE_SOURCE_DIR STREQUAL CMAKE_CURRENT_SOURCE_DIR OR LIBUNWIND_STANDALONE_BUILD)
   project(libunwind LANGUAGES NONE)
   llvm_enable_language_nolink(C CXX ASM)
 
   set(PACKAGE_NAME libunwind)
   set(PACKAGE_VERSION 14.0.0git)
   set(PACKAGE_STRING "${PACKAGE_NAME} ${PACKAGE_VERSION}")
   set(PACKAGE_BUGREPORT "[e-mail]")
 
-  # Add the CMake module path of libcxx so we can reuse HandleOutOfTreeLLVM.cmake
-  set(LIBUNWIND_LIBCXX_CMAKE_PATH "${LIBUNWIND_LIBCXX_PATH}/cmake/Modules")
-  list(APPEND CMAKE_MODULE_PATH "${LIBUNWIND_LIBCXX_CMAKE_PATH}")
-
   # In a standalone build, we don't have llvm to automatically generate the
   # llvm-lit script for us.  So we need to provide an explicit directory that
   # the configurator should write the script into.
   set(LIBUNWIND_STANDALONE_BUILD 1)
   set(LLVM_LIT_OUTPUT_DIR "${LIBUNWIND_BINARY_DIR}/bin")
 
   # Find the LLVM sources and simulate LLVM CMake options.
   include(HandleOutOfTreeLLVM)