[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.

cmake/Modules/AddCompilerRT.cmake

if(${type} STREQUAL "SHARED")
         # Ad-hoc sign the dylibs
         add_custom_command(TARGET ${libname}
           POST_BUILD  
           COMMAND codesign --sign - $<TARGET_FILE:${libname}>
           WORKING_DIRECTORY ${COMPILER_RT_LIBRARY_OUTPUT_DIR}
         )
       endif()
     endif()
-    install(TARGETS ${libname}
+    install(TARGETS ${libname} EXPORT LLVMRuntimes
       ARCHIVE DESTINATION ${COMPILER_RT_LIBRARY_INSTALL_DIR}
               ${COMPONENT_OPTION}
       LIBRARY DESTINATION ${COMPILER_RT_LIBRARY_INSTALL_DIR}
               ${COMPONENT_OPTION}
       RUNTIME DESTINATION ${COMPILER_RT_LIBRARY_INSTALL_DIR}
               ${COMPONENT_OPTION})
 
     # We only want to generate per-library install targets if you aren't using
foreach(libname ${libnames})
     if(APPLE)
       set_target_properties(${libname} PROPERTIES
       OSX_ARCHITECTURES "${LIB_ARCHS_${libname}}")
     endif()
 
     if(type STREQUAL "SHARED")
       rt_externalize_debuginfo(${libname})
     endif()
+    set_property(GLOBAL APPEND PROPERTY LLVM_RUNTIMES ${libname})
   endforeach()
   if(LIB_PARENT_TARGET)
     add_dependencies(${LIB_PARENT_TARGET} ${libnames})
   endif()
 endfunction()
 
 # when cross compiling, COMPILER_RT_TEST_COMPILER_CFLAGS help
 # in compilation and linking of unittests.