[TableGen] Fix a warning

This patch fixes:

  clang/utils/TableGen/ClangBuiltinTemplatesEmitter.cpp:25:8: error:
  'llvm::StringSet' may not intend to support class template argument
  deduction [-Werror,-Wctad-maybe-unsupported]

Author: kazutakahirata

clang/utils/TableGen/ClangBuiltinTemplatesEmitter.cpp

@@ -22,7 +22,7 @@ using namespace llvm;
 static std::string TemplateNameList;
 static std::string CreateBuiltinTemplateParameterList;
 
-static llvm::StringSet BuiltinClasses;
+static llvm::StringSet<> BuiltinClasses;
 
 namespace {
 struct ParserState {