drop more llvm:: prefixes on SmallString<>

More cleanup after r149799.

llvm-svn: 150380
diff --git a/clang/tools/libclang/CIndexDiagnostic.cpp b/clang/tools/libclang/CIndexDiagnostic.cpp
index ceb6405..1e6062e 100644
--- a/clang/tools/libclang/CIndexDiagnostic.cpp
+++ b/clang/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) {