Use SmallVectorImpl::iterator/const_iterator instead of SmallVector to avoid specifying the vector size.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@185606 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Support/YAMLTraits.cpp b/lib/Support/YAMLTraits.cpp
index 9da2aa7..1e42b63 100644
--- a/lib/Support/YAMLTraits.cpp
+++ b/lib/Support/YAMLTraits.cpp
@@ -322,7 +322,7 @@
 }
 
 bool Input::MapHNode::isValidKey(StringRef Key) {
-  for (SmallVector<const char *, 6>::iterator i = ValidKeys.begin(),
+  for (SmallVectorImpl<const char *>::iterator i = ValidKeys.begin(),
        End = ValidKeys.end(); i != End; ++i) {
     if (Key.equals(*i))
       return true;