Abseil LTS branch, Nov 2021

Abseil LTS 20211102

What's New:

• absl::Cord is now implemented as a b-tree. The new implementation offers improved performance in most workloads.
• absl::SimpleHexAtoi() has been added to strings library for parsing hexadecimal strings.

Breaking Changes:

• Bazel builds now depend on the bazelbuild/platforms repository. See Abseil's WORKSPACE file for an example of how to add this dependency.

Baseline: 2151058