Disambiguate clang::StringLiteral

...vs. recently introduced llvm::StringLiteral (llvm/ADT/StringRef.h)

Change-Id: I4d74546b0d1401a74b0c15368bbc93794ecd0b1d

compilerplugins/clang/stringconcat.cxx

@@ -37,7 +37,7 @@ Expr const * stripCtor(Expr const * expr) {
 }
 
 bool isStringLiteral(Expr const * expr) {
-    return isa<StringLiteral>(stripCtor(expr));
+    return isa<clang::StringLiteral>(stripCtor(expr));
 }
 
 class StringConcat: