drop more llvm:: prefixes on SmallString<>

More cleanup after r149799.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@150380 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/tools/libclang/CIndexDiagnostic.cpp b/tools/libclang/CIndexDiagnostic.cpp
index ceb6405..1e6062e 100644
--- a/tools/libclang/CIndexDiagnostic.cpp
+++ b/tools/libclang/CIndexDiagnostic.cpp
@@ -126,7 +126,7 @@
 
   CXDiagnosticSeverity Severity = clang_getDiagnosticSeverity(Diagnostic);
 
-  llvm::SmallString<256> Str;
+  SmallString<256> Str;
   llvm::raw_svector_ostream Out(Str);
   
   if (Options & CXDiagnostic_DisplaySourceLocation) {