Allow Bazel users to include Abseil headers with angle brackets.

Because we use Abseil with both CMake and Bazel, and we sometimes use Abseil from `/usr/include` (or `/usr/local/include` or some other $PREFIX) we need to always include Abseil headers with either quotes (`#include "absl/..."`) or angle brackets (`#include `).

We would prefer to include Abseil with angle brackets. In some platforms that allows us to treat warnings as errors (`-Werror -Wall -W...` or `/WX /W3`) even if the Abseil code is not "clean" for some warning. In general Abseil is very very clean, thanks! But there are a few exceptions, e.g. #582 .