[CMake] Collect target names in the global LLVM_RUNTIMES property

[phosek]

Summary

This allows exporting the runtimes targets using the CMake export.

[phosek]

Replaced by D49587.

lib/CMakeLists.txt

add_custom_command(TARGET cxx_shared POST_BUILD
     WORKING_DIRECTORY ${LIBCXX_BUILD_DIR}
   )
 endif()
 
 if (LIBCXX_INSTALL_LIBRARY)
   if (LIBCXX_INSTALL_EXPERIMENTAL_LIBRARY)
     set(experimental_lib cxx_experimental)
   endif()
-  install(TARGETS ${LIBCXX_TARGETS} ${experimental_lib}
+  install(TARGETS ${LIBCXX_TARGETS} ${experimental_lib} EXPORT LLVMRuntimes
     LIBRARY DESTINATION ${LIBCXX_INSTALL_PREFIX}lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT cxx
     ARCHIVE DESTINATION ${LIBCXX_INSTALL_PREFIX}lib${LIBCXX_LIBDIR_SUFFIX} COMPONENT cxx
     )
+  set_property(GLOBAL APPEND PROPERTY LLVM_RUNTIMES ${LIBCXX_TARGETS} ${experimental_lib})
   # NOTE: This install command must go after the cxx install command otherwise
   # it will not be executed after the library symlinks are installed.
   if (LIBCXX_ENABLE_SHARED AND LIBCXX_ENABLE_ABI_LINKER_SCRIPT)
     # Replace the libc++ filename with $<TARGET_LINKER_FILE:cxx>
     # after we required CMake 3.0.
     install(FILES "${LIBCXX_LIBRARY_DIR}/libc++${CMAKE_SHARED_LIBRARY_SUFFIX}"
       DESTINATION ${LIBCXX_INSTALL_PREFIX}lib${LIBCXX_LIBDIR_SUFFIX}
       COMPONENT libcxx)