[CMake][libcxx] Include AddLLVM needed for tests in the right context

[phosek]

Summary

AddLLVM is needed for several functions that are used in tests and
as such needs to be included from the right context which previously
wasn't the case.

[beanz]

LGTM.

CMakeLists.txt

 if(IS_DIRECTORY "${CMAKE_CURRENT_SOURCE_DIR}/test")
   add_subdirectory(test)
 endif()
 if (LIBCXX_INCLUDE_TESTS)
   add_subdirectory(lib/abi)
 endif()
 
 if (LIBCXX_STANDALONE_BUILD AND EXISTS "${LLVM_MAIN_SRC_DIR}/utils/llvm-lit")
+  include(AddLLVM) # for get_llvm_lit_path
   # Make sure the llvm-lit script is generated into the bin directory, and do
   # it after adding all tests, since the generated script will only work
   # correctly discovered tests against test locations from the source tree that
   # have already been discovered.
   add_subdirectory(${LLVM_MAIN_SRC_DIR}/utils/llvm-lit
                    ${CMAKE_CURRENT_BINARY_DIR}/llvm-lit)
 endif()
 
 if (LIBCXX_INCLUDE_DOCS)
   add_subdirectory(docs)
 endif()

test/CMakeLists.txt

 
 set(LIBCXX_TARGET_INFO "libcxx.test.target_info.LocalTI" CACHE STRING
     "TargetInfo to use when setting up test environment.")
 set(LIBCXX_EXECUTOR "None" CACHE STRING
     "Executor to use when running tests.")
 
 set(AUTO_GEN_COMMENT "## Autogenerated by libcxx configuration.\n# Do not edit!")
 
-configure_lit_site_cfg(
-  ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
-  ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg)
-
 set(LIBCXX_TEST_DEPS "")
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   set(LIBCXX_TEST_DEPS cxx_experimental)
 endif()
 
 if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)
   list(APPEND LIBCXX_TEST_DEPS cxx_external_threads)
 endif()
 
 if (LIBCXX_INCLUDE_TESTS)
-  include(AddLLVM) # for add_lit_testsuite
+  include(AddLLVM) # for configure_lit_site_cfg and add_lit_testsuit
+
+  configure_lit_site_cfg(
+    ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
+    ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg)
+
   add_lit_testsuite(check-cxx
     "Running libcxx tests"
     ${CMAKE_CURRENT_BINARY_DIR}
     DEPENDS cxx ${LIBCXX_TEST_DEPS})
 
   add_custom_target(check-libcxx DEPENDS check-cxx)
 endif()