DebugInfoFinder: Check for null imported entities

Don't use the accessors in `DIImportedEntity` on a null pointer.  (A WIP
patch to make `DIDescriptor` accessors more strict crashes here
otherwise.)

llvm-svn: 232833
diff --git a/llvm/lib/IR/DebugInfo.cpp b/llvm/lib/IR/DebugInfo.cpp
index 3c7cf9e..28244d6 100644
--- a/llvm/lib/IR/DebugInfo.cpp
+++ b/llvm/lib/IR/DebugInfo.cpp
@@ -674,6 +674,8 @@
       DIArray Imports = CU.getImportedEntities();
       for (unsigned i = 0, e = Imports.getNumElements(); i != e; ++i) {
         DIImportedEntity Import = DIImportedEntity(Imports.getElement(i));
+        if (!Import)
+          continue;
         DIDescriptor Entity = Import.getEntity().resolve(TypeIdentifierMap);
         if (Entity.isType())
           processType(DIType(Entity));