AsmPrinter: Return added DIE from DIE::addChild()

Change `DIE::addChild()` to return a reference to the just-added node,
and update consumers to use it directly.  An upcoming commit will
abstract away (and eventually change) the underlying storage of
`DIE::Children`.

llvm-svn: 238372
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index fc57f26..907f670 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -290,8 +290,7 @@
 DIE &DwarfUnit::createAndAddDIE(unsigned Tag, DIE &Parent, const DINode *N) {
   assert(Tag != dwarf::DW_TAG_auto_variable &&
          Tag != dwarf::DW_TAG_arg_variable);
-  Parent.addChild(make_unique<DIE>((dwarf::Tag)Tag));
-  DIE &Die = *Parent.getChildren().back();
+  DIE &Die = Parent.addChild(make_unique<DIE>((dwarf::Tag)Tag));
   if (N)
     insertDIE(N, &Die);
   return Die;