Merge #17682: util: Update tinyformat to upstream

978b255 util: Update tinyformat to upstream (Wladimir J. van der Laan)

Pull request description:

  Last update was in 2017.
  Updates tinyformat to upstream commit  c42f/tinyformat@705e3f4.
  Re-apply (and mark) bitcoin core specific changes.

  No changes that affect our use, as far as I can see, but this gets rid of the gcc `-Wimplicit-fallthrough` warnings, at least.

ACKs for top commit:
  MarcoFalke:
    ACK 978b255, extracted our patches based on the last update, did the update to v2.3.0 myself and re-applied the patches. Only diff is NULL/nullptr and explicit 🔝

Tree-SHA512: 2ba09e1095878d088520f379d545b40c7286ef199ecbbc17fdd5c85bca447d9b0c7a1829d4038bb6d432cd1ff92ad7bba75c0f2f96c71aeb6fa6031002f1ea1d

Author: MarcoFalke