Add `location_from_id` provider-defined function

[SarahFrench]

This PR adds support for a new provider-defined function:

• location_from_id

There are unit and acceptance tests for both of these, and documentation

Note about testing

Currently, testing this PR requires downloading an alpha release of TF 1.8.0 and running the acceptance test locally. The unit tests should work fine because they don't use a TF binary like acc tests do, and the dependencies on this branch include provider function support.

I'm working on adding automated testing for the feature branch and provider-defined function PR here: https://github.com/hashicorp/terraform-provider-google/issues/17451

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Release Note Template for Downstream PRs (will be copied)

provider: added provider-defined function `location_from_id` for retrieving the location from a resource's self link or id

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[SarahFrench]

Thanks Mau!