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/TextDiagnosticPrinter.cpp b/lib/Frontend/TextDiagnosticPrinter.cpp
index b5d0bcb..465dcad 100644
--- a/lib/Frontend/TextDiagnosticPrinter.cpp
+++ b/lib/Frontend/TextDiagnosticPrinter.cpp
@@ -132,7 +132,7 @@
 
   // Render the diagnostic message into a temporary buffer eagerly. We'll use
   // this later as we print out the diagnostic to the terminal.
-  llvm::SmallString<100> OutStr;
+  SmallString<100> OutStr;
   Info.FormatDiagnostic(OutStr);
 
   llvm::raw_svector_ostream DiagMessageStream(OutStr);