readability-const-return-type: don't diagnose a template function returning T,...

readability-const-return-type: don't diagnose a template function returning T, even if sometimes instantiated with e.g. T = const int.

It's not really a readability problem since there's no `const` to read at the declaration site, and returning std::remove_const_t<T> instead usually only hurts readability.

Reviewed By: ymandel

Differential Revision: https://reviews.llvm.org/D140434