[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.

src/CMakeLists.txt

 # Add a meta-target for both libraries.
 add_custom_target(cxxabi DEPENDS ${LIBCXXABI_TARGETS})
 
 if (LIBCXXABI_INSTALL_LIBRARY)
   install(TARGETS ${LIBCXXABI_TARGETS}
     LIBRARY DESTINATION ${LIBCXXABI_INSTALL_PREFIX}lib${LIBCXXABI_LIBDIR_SUFFIX} COMPONENT cxxabi
     ARCHIVE DESTINATION ${LIBCXXABI_INSTALL_PREFIX}lib${LIBCXXABI_LIBDIR_SUFFIX} COMPONENT cxxabi
     )
+  set_property(GLOBAL APPEND PROPERTY LLVM_RUNTIMES ${LIBCXXABI_TARGETS})
 endif()
 
 if (NOT CMAKE_CONFIGURATION_TYPES AND LIBCXXABI_INSTALL_LIBRARY)
   add_custom_target(install-cxxabi
     DEPENDS cxxabi
     COMMAND "${CMAKE_COMMAND}"
             -DCMAKE_INSTALL_COMPONENT=cxxabi
             -P "${LIBCXXABI_BINARY_DIR}/cmake_install.cmake")