Revert "[IR] Remove the DIExpression field from DIGlobalVariable."

This reverts commit 289920 (again).
I forgot to implement a Bitcode upgrade for the case where a DIGlobalVariable
has not DIExpression. Unfortunately it is not possible to safely upgrade
these variables without adding a flag to the bitcode record indicating which
version they are.
My plan of record is to roll the planned follow-up patch that adds a
unit: field to DIGlobalVariable into this patch before recomitting.
This way we only need one Bitcode upgrade for both changes (with a
version flag in the bitcode record to safely distinguish the record
formats).

Sorry for the churn!

llvm-svn: 289982

llvm/lib/Analysis/ModuleDebugInfoPrinter.cpp

@@ -91,8 +91,7 @@ void ModuleDebugInfoPrinter::print(raw_ostream &O, const Module *M) const {
     O << '\n';
   }
 
-  for (auto GVU : Finder.global_variables()) {
-    const auto *GV = GVU->getVariable();
+  for (const DIGlobalVariable *GV : Finder.global_variables()) {
     O << "Global variable: " << GV->getName();
     printFile(O, GV->getFilename(), GV->getDirectory(), GV->getLine());
     if (!GV->getLinkageName().empty())