Assert that DIEEntries are constructed with non-null DIEs

This just brings a crash a little further forward from DWARF emission to
DIE construction to make errors easier to diagnose.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@181748 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/AsmPrinter/DIE.h

@@ -325,7 +325,9 @@ namespace llvm {
   class DIEEntry : public DIEValue {
     DIE *const Entry;
   public:
-    explicit DIEEntry(DIE *E) : DIEValue(isEntry), Entry(E) {}
+    explicit DIEEntry(DIE *E) : DIEValue(isEntry), Entry(E) {
+      assert(E && "Cannot construct a DIEEntry with a null DIE");
+    }
 
     DIE *getEntry() const { return Entry; }