[NFC] Turn "load dependent files" boolean into an enum

This is an NFC commit to refactor the "load dependent files" parameter
from a boolean to an enum value. We want to be able to specify a
default, in which case we decide whether or not to load the dependent
files based on whether the target is an executable or not (i.e. a
dylib).

This is a dependency for D51934.

Differential revision: https://reviews.llvm.org/D51859

llvm-svn: 342633
diff --git a/lldb/source/Core/DynamicLoader.cpp b/lldb/source/Core/DynamicLoader.cpp
index 16f1ffc..576ec1e 100644
--- a/lldb/source/Core/DynamicLoader.cpp
+++ b/lldb/source/Core/DynamicLoader.cpp
@@ -101,8 +101,7 @@
         if (executable.get() != target.GetExecutableModulePointer()) {
           // Don't load dependent images since we are in dyld where we will
           // know and find out about all images that are loaded
-          const bool get_dependent_images = false;
-          target.SetExecutableModule(executable, get_dependent_images);
+          target.SetExecutableModule(executable, eLoadDependentsNo);
         }
       }
     }