DWARF: Generate the address lookup table from the DIE tree if .debug_aranges is not available.

Ported from LLDB.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@139732 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/DebugInfo/DWARFContext.cpp b/lib/DebugInfo/DWARFContext.cpp
index c034bb3..3359ae3 100644
--- a/lib/DebugInfo/DWARFContext.cpp
+++ b/lib/DebugInfo/DWARFContext.cpp
@@ -46,6 +46,8 @@
 
   Aranges.reset(new DWARFDebugAranges());
   Aranges->extract(arangesData);
+  if (Aranges->isEmpty()) // No aranges in file, generate them from the DIEs.
+    Aranges->generate(this);
   return Aranges.get();
 }