Make ConstraintKind an enum class.

llvm-svn: 276697
diff --git a/lld/ELF/LinkerScript.cpp b/lld/ELF/LinkerScript.cpp
index b1779b2..2c8614d 100644
--- a/lld/ELF/LinkerScript.cpp
+++ b/lld/ELF/LinkerScript.cpp
@@ -692,9 +692,9 @@
 
   // Parse constraints.
   if (skip("ONLY_IF_RO"))
-    Cmd->Constraint = ReadOnly;
+    Cmd->Constraint = ConstraintKind::ReadOnly;
   if (skip("ONLY_IF_RW"))
-    Cmd->Constraint = ReadWrite;
+    Cmd->Constraint = ConstraintKind::ReadWrite;
   expect("{");
 
   while (!Error && !skip("}")) {