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.
`partial_apply` can be rewritten to `thin_to_thick_function` only if the
specialized callee is `@convention(thin)`.
This condition is newly exercised by the differentiation transform:
`{JVP,VJP}Emitter::visitApplyInst` generates argument-less `partial_apply`
with `@convention(method)` callees.
Resolves SR-12732.