DebugInfo: New metadata representation for global variables.

This patch reverses the edge from DIGlobalVariable to GlobalVariable.
This will allow us to more easily preserve debug info metadata when
manipulating global variables.

Fixes PR30362. A program for upgrading test cases is attached to that
bug.

Differential Revision: http://reviews.llvm.org/D20147

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@281284 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/IR/DIBuilder.h

@@ -454,20 +454,21 @@ namespace llvm {
     /// \param Ty          Variable Type.
     /// \param isLocalToUnit Boolean flag indicate whether this variable is
     ///                      externally visible or not.
-    /// \param Val         llvm::Value of the variable.
+    /// \param Expr        The location of the global relative to the attached
+    ///                    GlobalVariable.
     /// \param Decl        Reference to the corresponding declaration.
     DIGlobalVariable *createGlobalVariable(DIScope *Context, StringRef Name,
                                            StringRef LinkageName, DIFile *File,
                                            unsigned LineNo, DIType *Ty,
                                            bool isLocalToUnit,
-                                           llvm::Constant *Val,
+                                           DIExpression *Expr = nullptr,
                                            MDNode *Decl = nullptr);
 
     /// Identical to createGlobalVariable
     /// except that the resulting DbgNode is temporary and meant to be RAUWed.
     DIGlobalVariable *createTempGlobalVariableFwdDecl(
         DIScope *Context, StringRef Name, StringRef LinkageName, DIFile *File,
-        unsigned LineNo, DIType *Ty, bool isLocalToUnit, llvm::Constant *Val,
+        unsigned LineNo, DIType *Ty, bool isLocalToUnit, DIExpression *Expr,
         MDNode *Decl = nullptr);
 
     /// Create a new descriptor for an auto variable.  This is a local variable
@@ -514,6 +515,13 @@ namespace llvm {
     DIExpression *createBitPieceExpression(unsigned OffsetInBits,
                                            unsigned SizeInBits);
 
+    /// Create an expression for a variable that does not have an address, but
+    /// does have a constant value.
+    DIExpression *createConstantValueExpression(uint64_t Val) {
+      return DIExpression::get(
+          VMContext, {dwarf::DW_OP_constu, Val, dwarf::DW_OP_stack_value});
+    }
+
     /// Create a new descriptor for the specified subprogram.
     /// See comments in DISubprogram* for descriptions of these fields.
     /// \param Scope         Function scope.