Add warning about redundant moves

Summary: As per title. This option becomes enabled by default in GCC9, so we might as well do it all the time.

Test Plan:

make check

Verify that D4035 is the only warning of the sort.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D4036