Add README files explaining the differentiable programming test suite.
Add lit.local.cfg files so individual tests do not need to add
`REQUIRES: asserts` in these directories:
- test/AutoDiff/compiler_crashers
- test/AutoDiff/compiler_crashers_fixed
Motivation: it is important for compiler crasher tests to require assertions
enabled, because many of these tests crash on compiler assertions.
* The update in `SILFunctionType.cpp` fixes SR-12641 by making address-only parameters/results in differentials have indirect convention.
* I updated the crasher test to use a resilient struct defined in the test, instead of `Tracked<Float>`, so that the test does not need to depend on `DifferentiationUnittest`.
* The update in `VJPEmitter.cpp` fixes a similar issue with pullbacks that I discovered while investigating.
* I added code that exposes this new issue to the SR-12641 crasher test.
