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/Frontend/TextDiagnosticBuffer.cpp b/lib/Frontend/TextDiagnosticBuffer.cpp
index f8ea9f1..57105f1 100644
--- a/lib/Frontend/TextDiagnosticBuffer.cpp
+++ b/lib/Frontend/TextDiagnosticBuffer.cpp
@@ -24,7 +24,7 @@
   // Default implementation (Warnings/errors count).
   DiagnosticConsumer::HandleDiagnostic(Level, Info);
 
-  llvm::SmallString<100> Buf;
+  SmallString<100> Buf;
   Info.FormatDiagnostic(Buf);
   switch (Level) {
   default: llvm_unreachable(