Clang warns when "return std::move(x)" is needed to elide a copy,
but the call to std::move() is missing. We disabled the warning during
the migration to clang. This patch re-enables the warning and fixes
the places it points out, usually by adding std::move() and in one
place by converting the returned variable from a reference to a local,
so normal copy elision can take place.
Closes#8739
0acf5bfca6