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.
Previously, two conditions were necessary to enable differentiable programming:
- Using the `-enable-experimental-differentiable-programming` frontend flag.
- Importing the `_Differentiation` module.
Importing the `_Differentiation` module is the true condition because it
contains the required compiler-known `Differentiable` protocol. The frontend
flag is redundant and cumbersome.
Now, the frontend flag is removed.
Importing `_Differentiation` is the only condition.
Remove meaningless `assert(false)` assertion from
`hasOverridingDifferentiableAttribute` in `TypeCheckDeclOverride.cpp`.
The assertion served no purpose and can safely be removed.
Resolves TF-1167. Add test.
