build: Enable -Wredundant-decls where available

Summary:
Backport of core PR13899
https://github.com/bitcoin/bitcoin/pull/13899/files

Only commit f04bb13 is applicable to us thanks to D45.

Test Plan:

make check
ninja check

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D3823