Abseil LTS 20200923

What's New:

• absl::StatusOr<T> has been released. See our blog post for more information.
• Abseil Flags reflection interfaces have been released.
• Abseil Flags memory usage has been significantly optimized.
• Abseil now supports a "hardened" build mode. This build mode enables runtime checks that guard against programming errors that may lead to security vulnerabilities.

Notable Fixes:

• Sanitizer dynamic annotations like AnnotateRWLockCreate that are also defined by the compiler sanitizer implementation are no longer also defined by Abseil.
• Sanitizer macros are now prefixed with ABSL_ to avoid naming collisions.
• Sanitizer usage is now automatically detected and no longer requires macros like ADDRESS_SANITIZER to be defined on the command line.

Breaking Changes:

• Abseil no longer contains a dynamic_annotations library. Users using a supported build system (Bazel or CMake) are unaffected by this, but users manually specifying link libraries may get an error about a missing linker input.

Baseline: 7680a5f8efe32de4753baadbd63e74e59d95bac1 Cherry picks: None