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/InitPreprocessor.cpp b/lib/Frontend/InitPreprocessor.cpp
index 93c9989..f0cf7f4 100644
--- a/lib/Frontend/InitPreprocessor.cpp
+++ b/lib/Frontend/InitPreprocessor.cpp
@@ -127,7 +127,7 @@
                "1.79769313486231580793728971405301e+308L",
                "1.18973149535723176508575932662800702e+4932L");
 
-  llvm::SmallString<32> DefPrefix;
+  SmallString<32> DefPrefix;
   DefPrefix = "__";
   DefPrefix += Prefix;
   DefPrefix += "_";