[cmake] Only set deps for an ExternalProject if the type is executable or library

[lanza]

Summary

cmake fails with an error when attempting to evaluate $<TARGET_FILE:tgt>
where tgt is defined via an add_custom_target and thus the TYPE
is UTILITY. Requesting a TARGET_FILE only works on an EXECUTABLE
or one of a few differetnt types of X_LIBRARY (e.g. added via
add_library or add_executable). The logic as implemented in cmake
is below:

enum TargetType
{
  EXECUTABLE,
  STATIC_LIBRARY,
  SHARED_LIBRARY,
  MODULE_LIBRARY,
  OBJECT_LIBRARY,
  UTILITY,
  GLOBAL_TARGET,
  INTERFACE_LIBRARY,
  UNKNOWN_LIBRARY
};

if (target->GetType() >= cmStateEnums::OBJECT_LIBRARY &&
    target->GetType() != cmStateEnums::UNKNOWN_LIBRARY) {
  ::reportError(context, content->GetOriginalExpression(),
                "Target \"" + name +
                  "\" is not an executable or library.");
  return nullptr;
}

This has always been the case back to at least 3.12 (furthest I
checked) but this is causing a new failure in cmake 3.17 while
evaluating ExternalProjectAdd.

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

if(NOT ARG_TOOLCHAIN_TOOLS)
     if(NOT _cmake_system_name STREQUAL Darwin)
       # TODO: These tools don't fully support Mach-O format yet.
       list(APPEND ARG_TOOLCHAIN_TOOLS llvm-objcopy llvm-strip)
     endif()
   endif()
   foreach(tool ${ARG_TOOLCHAIN_TOOLS})
     if(TARGET ${tool})
       list(APPEND TOOLCHAIN_TOOLS ${tool})
+
+      # $<TARGET_FILE:tgt> only works on add_executable or add_library targets
+      # The below logic mirrors cmake's own implementation
+      get_target_property(target_type "${tool}" TYPE)
+      if(NOT target_type STREQUAL "OBJECT_LIBRARY" AND
+         NOT target_type STREQUAL "UTILITY" AND
+         NOT target_type STREQUAL "GLOBAL_TARGET" AND
+         NOT target_type STREQUAL "INTERFACE_LIBRARY")
         list(APPEND TOOLCHAIN_BINS $<TARGET_FILE:${tool}>)
       endif()
+
+    endif()
   endforeach()
 
   if(NOT ARG_RUNTIME_LIBRARIES)
     set(ARG_RUNTIME_LIBRARIES compiler-rt libcxx)
   endif()
   foreach(lib ${ARG_RUNTIME_LIBRARIES})
     if(TARGET ${lib})
       list(APPEND RUNTIME_LIBRARIES ${lib})