[llgo] debug: create replaceable type through DIBuilder

[axw]

Summary

llgo was asserting in DebugInfo, which was interpreting
the temporary MDNodes we were creating as DIScopes instead
of DITypes (in DIScope::getRef).

This proposal changes llgo to use DIBuilder's
createReplaceableCompositeType method to create a DIType
that can be RAUW'd.

[pcc]

LGTM

debug/debug.go

return d.builder.CreateStructType(d.cu, llvm.DIStructType{
     Name:        name,
     SizeInBits:  uint64(d.sizes.Sizeof(t) * 8),
     AlignInBits: uint64(d.sizes.Alignof(t) * 8),
     Elements:    members,
   })
 }
 
 func (d *DIBuilder) descriptorNamed(t *types.Named) llvm.Metadata {
-  // Create a placeholder for the named type, to terminate cycles.
-  placeholder := llvm.GlobalContext().TemporaryMDNode(nil)
-  d.types.Set(t, placeholder)
   var diFile llvm.Metadata
   var line int
   if file := d.fset.File(t.Obj().Pos()); file != nil {
     line = file.Line(t.Obj().Pos())
     diFile = d.getFile(file)
   }
+
+  // Create a placeholder for the named type, to terminate cycles.
+  name := t.Obj().Name()
+  placeholder := d.builder.CreateReplaceableCompositeType(d.scope(), llvm.DIReplaceableCompositeType{
+    Tag:  dwarf.TagStructType,
+    Name: name,
+    File: diFile,
+    Line: line,
+  })
+  d.types.Set(t, placeholder)
+
   typedef := d.builder.CreateTypedef(llvm.DITypedef{
     Type: d.DIType(t.Underlying()),
-    Name: t.Obj().Name(),
+    Name: name,
     File: diFile,
     Line: line,
   })
   placeholder.ReplaceAllUsesWith(typedef)
   return typedef
 }
 
 func (d *DIBuilder) descriptorArray(t *types.Array, name string) llvm.Metadata {