This replaces gflags. Added TODOs where use of `absl::Duration` or `absl::FlagSaver` might be preferred in follow-up cleanup. Fixes#24493.
This reverts commit da66b7d14e.
NEW:
* Adds references to `absl/flags/declare.h`, new to LTS 2020923.2 imported in commit 5b43440.
* Works around MSVC 2017 compiler error with large help text on flags by reducing the help text.
This is for compatibility with IPv6-only environments; see the Google
bug b/32531998.
Note that InteropClient.cs and http2_client.cc don't appear to be tested
by our internal branch, but I've included them for consistency.