import abseil::optional

[zxu123]

Based on abseil version bf7fc99, the same version as the other imported abseil libraries.

Will eventually merge to master; right now into the branch abseil-mirror for easy review.

[wilhuff]

This should not be merged into abseil-mirror.

You should do something like this

# create a new branch from master
git checkout -b import-optional master

# merge in the updated abseil-mirror branch that contains the pristine sources
git merge abseil-mirror

# apply the changes that make optional buildable
git cherry-pick bcf8dc8ff04ecf5a29d67fc1855b8e0ec8e33600

[wilhuff]

I've essentially done this (plus brought in the optional_test.cc and the CMakeLists.txt on the upstream branch) in #1112. Let's continue there.