[IR] Remove the DIExpression field from DIGlobalVariable.

This patch implements PR31013 by introducing a
DIGlobalVariableExpression that holds a pair of DIGlobalVariable and
DIExpression.

Currently, DIGlobalVariables holds a DIExpression. This is not the
best way to model this:

(1) The DIGlobalVariable should describe the source level variable,
    not how to get to its location.

(2) It makes it unsafe/hard to update the expressions when we call
    replaceExpression on the DIGLobalVariable.

(3) It makes it impossible to represent a global variable that is in
    more than one location (e.g., a variable with multiple
    DW_OP_LLVM_fragment-s).  We also moved away from attaching the
    DIExpression to DILocalVariable for the same reasons.

<rdar://problem/29250149>
https://llvm.org/bugs/show_bug.cgi?id=31013
Differential Revision: https://reviews.llvm.org/D26769

llvm-svn: 289902

llvm/lib/Analysis/ModuleDebugInfoPrinter.cpp

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