Add unsigned to character buffers to ensure they can provide storage (https://eel.is/c++draft/intro.object#3)

[CJ-Johnson]

I just learned today that char cannot "provide storage" in the way I previously thought. Only std::byte and unsigned char are allowed. So I decided to fix my past mistakes in absl::InlinedVector and absl::Cleanup as well as any others I could find in Abseil. See: https://eel.is/c++draft/intro.object#3

[derekmauro]

Thanks! This currently doesn't build in any configuration. Can you please try building the code?

[CJ-Johnson]

@derekmauro Thanks for the headsup! I attempted to build it locally but ran into an issue. Basically, my Apple clang/libc++ version is too old, I think. I reinstalled xcode command line tools as well as llvm via homebrew, but the issue persists. You can see the output here: https://pastebin.com/ZV5VJAiR

Are you able to share the Kokoro output with me? That way I can see what the actual failure is as opposed to the unrelated issue I see locally.

[CJ-Johnson]

@derekmauro Actually after updating Xcode and running bazel clean --expunge I was able to get it building locally. Only issue now is the random benchmarks keep failing on me. It's not clear that my changes are causing that. Can you take a look?

[CJ-Johnson]

@derekmauro Also while I have you, are you able to review and land abseil/abseil.github.io#501 ? Thanks!

[derekmauro]

Thanks. It was actually easier for me to make the simple fix myself. Merged.

For abseil/abseil.github.io#501, I'm going to have to take a look to see what the procedure is to update the documentation.

[CJ-Johnson]

Awesome thanks!