Basic: import SmallString<> into clang namespace

(I was going to fix the TODO about DenseMap too, but
that would break self-host right now. See PR11922.)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@149799 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Rewrite/HTMLRewrite.cpp b/lib/Rewrite/HTMLRewrite.cpp
index 77c9969..44d635a 100644
--- a/lib/Rewrite/HTMLRewrite.cpp
+++ b/lib/Rewrite/HTMLRewrite.cpp
@@ -210,7 +210,7 @@
 
 static void AddLineNumber(RewriteBuffer &RB, unsigned LineNo,
                           unsigned B, unsigned E) {
-  llvm::SmallString<256> Str;
+  SmallString<256> Str;
   llvm::raw_svector_ostream OS(Str);
 
   OS << "<tr><td class=\"num\" id=\"LN"