[llgo] Remove support for LLVM attributes

[meadori]

Summary

llgo supports the application of LLVM attributes to global objects
and functions. This "feature" is undocumented and untested. As
discusses in D27442, it should be removed.

[pcc]

LGTM

irgen/attribute.go

func parseAttribute(line string) Attribute {
   } else {
     key, value = line[:colon], line[colon+1:]
   }
   switch key {
   case "linkage":
     return parseLinkageAttribute(value)
   case "name":
     return nameAttribute(strings.TrimSpace(value))
-  case "attr":
-    return parseLLVMAttribute(strings.TrimSpace(value))
   case "thread_local":
     return tlsAttribute{}
   default:
     // FIXME decide what to do here. return error? log warning?
     panic("unknown attribute key: " + key)
   }
   return nil
 }
if !curr.IsNil() && curr != v {
       curr.ReplaceAllUsesWith(llvm.ConstBitCast(v, curr.Type()))
     }
     v.SetName(name)
   } else {
     v.SetName(string(a))
   }
 }
 
-func parseLLVMAttribute(value string) llvmAttribute {
-  var result llvmAttribute
-  value = strings.Replace(value, ",", " ", -1)
-  for _, field := range strings.Fields(value) {
-    switch strings.ToLower(field) {
-    case "noreturn":
-    case "nounwind":
-    case "noinline":
-    case "alwaysinline":
-      kind := llvm.AttributeKindID(strings.ToLower(field))
-      result.AttrKinds = append(result.AttrKinds, kind)
-    }
-  }
-  return result
-}
-
-type llvmAttribute struct {
-  AttrKinds []uint
-}
-
-func (a llvmAttribute) Apply(v llvm.Value) {
-  ctx := v.GlobalParent().Context()
-  if !v.IsAFunction().IsNil() {
-    for _, kind := range a.AttrKinds {
-      attr := ctx.CreateEnumAttribute(kind, 0)
-      v.AddFunctionAttr(attr)
-    }
-  }
-}
-
 type tlsAttribute struct{}
 
 func (tlsAttribute) Apply(v llvm.Value) {
   v.SetThreadLocal(true)
 }