DebugInfo: Partial revert r209984 due to more cases where abstract DbgVariables do not have associated DIEs.

Along with a test case to demonstrate that due to inlining order there
are cases where abstract variable DIEs are not constructed since the
abstract subprogram was built due to a previous inlining that optimized
away those variables. This produces incorrect debug info (the 'missing'
abstract variable causes the inlined instance of that variable to be
emitted with a full description (name, line, file) rather than
referencing the abstract origin), but this commit at least ensures that
it doesn't crash...

llvm-svn: 210143
diff --git a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
index fddc7fa..2325ab8 100644
--- a/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
+++ b/llvm/lib/CodeGen/AsmPrinter/DwarfUnit.cpp
@@ -1781,7 +1781,11 @@
 
   // Define variable debug information entry.
   auto VariableDie = make_unique<DIE>(DV.getTag());
-  if (DbgVariable *AbsVar = DV.getAbstractVariable())
+  DbgVariable *AbsVar = DV.getAbstractVariable();
+  // FIXME: any missing abstract variable missing a DIE will result in incorrect
+  // DWARF. More details in test/DebugInfo/missing-abstract-variable.ll for an
+  // example of why this is happening.
+  if (AbsVar && AbsVar->getDIE())
     addDIEEntry(*VariableDie, dwarf::DW_AT_abstract_origin, *AbsVar->getDIE());
   else {
     if (!Name.empty())