[CMake] Support externalizing debug info on non-Darwin platforms

[phosek]

Summary

On other platorms, we use objcopy to export the debug info.

[phosek]

ping?

[beanz]

A few small comments below. Otherwise LGTM.

cmake/modules/AddLLVM.cmake

1398

This block should be unneeded because you're already setting strip_command above.

1410–1414

This also should be unneeded.

[beanz]

LGTM! Thanks!

cmake/modules/AddLLVM.cmake

 endfunction()
 
 function(llvm_externalize_debuginfo name)
   if(NOT LLVM_EXTERNALIZE_DEBUGINFO)
     return()
   endif()
 
   if(NOT LLVM_EXTERNALIZE_DEBUGINFO_SKIP_STRIP)
+    if(APPLE)
       set(strip_command COMMAND xcrun strip -Sxl $<TARGET_FILE:${name}>)
+    else()
+      set(strip_command COMMAND strip -gx $<TARGET_FILE:${name}>)
+    endif()
   endif()
 
   if(APPLE)
     if(CMAKE_CXX_FLAGS MATCHES "-flto"
       OR CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE} MATCHES "-flto")
 
       set(lto_object ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/${name}-lto.o)
       set_property(TARGET ${name} APPEND_STRING PROPERTY
         LINK_FLAGS " -Wl,-object_path_lto,${lto_object}")
     endif()
     add_custom_command(TARGET ${name} POST_BUILD
       COMMAND xcrun dsymutil $<TARGET_FILE:${name}>
       ${strip_command}
       )
   else()
-    message(FATAL_ERROR "LLVM_EXTERNALIZE_DEBUGINFO isn't implemented for non-darwin platforms!")
+    add_custom_command(TARGET ${name} POST_BUILD
+      COMMAND objcopy --only-keep-debug $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.debug
+      ${strip_command} -R .gnu_debuglink
+      COMMAND objcopy --add-gnu-debuglink=$<TARGET_FILE:${name}>.debug $<TARGET_FILE:${name}>
+      )
   endif()
 endfunction()
 
 function(llvm_setup_rpath name)
   if(CMAKE_INSTALL_RPATH)
     return()
   endif()