IR/Attributes: Provide EmptyKey and TombstoneKey in part of enum, as workaround for gcc-4.4 take #2.

I will investigate, later, what was wrong. I am too tired for now.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@171611 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/IR/Attributes.cpp b/lib/IR/Attributes.cpp
index 84f90ad..039e036 100644
--- a/lib/IR/Attributes.cpp
+++ b/lib/IR/Attributes.cpp
@@ -426,6 +426,8 @@
 uint64_t AttributeImpl::getAttrMask(Attribute::AttrKind Val) {
   switch (Val) {
   case Attribute::EndAttrKinds:    break;
+  case Attribute::EmptyKey:        break;
+  case Attribute::TombstoneKey:    break;
   case Attribute::None:            return 0;
   case Attribute::ZExt:            return 1 << 0;
   case Attribute::SExt:            return 1 << 1;