Revive lit tests

[lorentey]

This repository contains a couple of lit tests that look at generated code to catch potential codegen issues and to verify that constant folding gets rid of switch statements over memory ordering enumerations. These do not cover all supported atomic types and they aren't currently configured for execution.

• Set up a lit configuration that allows us to easily execute these tests. This does not need to be integrated into the package manager.
• Update and extend the existing tests to look at double-wide atomics, atomic booleans and other features that aren't currently covered.