[libcxx][CMake] Fix libc++ build when no LLVM source tree is available.

[vsapsai]

Summary

Fixes error

CMake Error at test/CMakeLists.txt:52 (configure_lit_site_cfg):
Unknown CMake command "configure_lit_site_cfg".

configure_lit_site_cfg is defined in AddLLVM module and not available
without LLVM source tree. Revert back to configure_file (reverts part of
r313643). It is possible as libcxx/test/lit.site.cfg.in doesn't use
latest lit capabilities.

Tested with

• in-tree libcxx - no change in generated lit.site.cfg and running tests;
• standalone libcxx with lit checked out next to it - no CMake errors.

If there are other tricky configurations worth testing, let me know.

rdar://problem/35303262

[vsapsai]

Fixed by changing libcxx build configuration, no CMake change required.

libcxx/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(
+configure_file(
   ${CMAKE_CURRENT_SOURCE_DIR}/lit.site.cfg.in
-  ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg)
+  ${CMAKE_CURRENT_BINARY_DIR}/lit.site.cfg
+  @ONLY)
 
 set(LIBCXX_TEST_DEPS "")
 
 if (LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY)
   set(LIBCXX_TEST_DEPS cxx_experimental)
 endif()
 
 if (LIBCXX_BUILD_EXTERNAL_THREAD_LIBRARY)