Abseil LTS branch, June 2022

Abseil LTS 20220623

What's New:

• Added absl::AnyInvocable, a move-only function type.
• Added absl::CordBuffer, a type for buffering data for eventual inclusion an absl::Cord, which is useful for writing zero-copy code.
• Added support for command-line flags of type absl::optional<T>.

Breaking Changes:

• CMake builds now use the flag ABSL_BUILD_TESTING (default: OFF) to control whether or not unit tests are built.
• The ABSL_DEPRECATED macro now works with the GCC compiler. GCC users that are experiencing new warnings can use -Wno-deprecated-declatations silence the warnings or use -Wno-error=deprecated-declarations to see warnings but not fail the build.
• ABSL_CONST_INIT uses the C++20 keyword constinit when available. Some compilers are more strict about where this keyword must appear compared to the pre-C++20 implementation.
• Bazel builds now depend on the bazelbuild/bazel-skylib repository. See Abseil's WORKSPACE file for an example of how to add this dependency.

Other:

• This will be the last release to support C++11. Future releases will require at least C++14.

Baseline: 273292d