Abseil LTS 20230802.1

What's New:

Added the nullability library for designating the expected nullability of pointers. Currently these serve as annotations only, but it is expected that compilers will one day be able to use these annotations for diagnostic purposes.
Added the prefetch library as a portable layer for moving data into caches before it is read.
Abseil's hash tables now detect many more programming errors in debug and sanitizer builds.
Abseil's synchronization objects now differentiate absolute waits (when passed an absl::Time) from relative waits (when passed an absl::Duration) when the underlying platform supports differentiating these cases. This only makes a difference when system clocks are adjusted.
Abseil's flag parsing library includes additional methods that make it easier to use when another library also expects to be able to parse flags.
absl::string_view is now available as a smaller target, @com_google_absl//absl/strings:string_view, so that users may use this library without depending on the much larger @com_google_absl//absl/strings target.

Baseline: fdf5be1
Cherry pick: 659b77b
Cherry pick: fb3621f (Patch 1)