Use 'auto' to avoid implicit copies.

td_type is std::pair<std::string, std::string>, but the map returns
elements of std::pair<const std::string, std::string>. In well-designed
languages like C++ that yields an implicit copy perfectly hidden by
constref's lifetime extension. Just use auto, the typedef obscured the
real type anyways.

Found with a little help from clang-tidy's
performance-implicit-cast-in-loop.

llvm-svn: 272519