[AutoDiff] First cut of coroutines differentiation

[asl]

This PR implements first set of changes required to support autodiff for coroutines. It mostly targeted to _modify accessors in standard library (and beyond), but overall implementation is quite generic.

There are some specifics of implementation and known limitations:

• Only @yield_once coroutines are naturally supported
• VJP is a coroutine itself: it yields the results and returns a pullback closure as a normal return. This allows us to capture values produced in resume part of a coroutine (this is required for defers and other cleanups / commits)
• Pullback is a coroutine, we assume that coroutine cannot abort and therefore we execute the original coroutine in reverse from return via yield and then back to the entry
• It seems there is no semantically sane way to support _read coroutines (as we will need to "accept" adjoints via yields), therefore only coroutines with inout yields are supported (_modify accessors). Pullbacks of such coroutines take adjoint buffer as input argument, yield this buffer (to accumulate adjoint values in the caller) and finally return the adjoints indirectly.
• Coroutines (as opposed to normal functions) are not first-class values: there is no AST type for them, one cannot e.g. store them into tuples, etc. So, everywhere where AST type is required, we have to hack around.
• As there is no AST type for coroutines, there is no way one could register custom derivative for coroutines. So far only compiler-produced derivatives are supported
• There are lots of common things wrt normal function apply's, but still there are subtle but important differences. I tried to organize the code to enable code reuse, still it was not always possible, so some code duplication could be seen
• The order of how pullback closures are produced in VJP is a bit different: for normal apply's VJP produces both value and pullback closure via a single nested VJP apply. This is not so anymore with coroutine VJP's: yielded values are produced at begin_apply site and pullback closure is available only from end_apply, so we need to track the order in which pullbacks are produced (and arrange consumption of the values accordingly – effectively delay them)
• On the way some complementary changes were required in e.g. mangler / demangler

This patch covers the generation of derivatives up to SIL level, however, it is not enough as codegen of partial_apply of a coroutine is completely broken. The fix for this will be submitted separately as it is not directly autodiff-related.

Some diagnostics were contributed by @asavonic

[asl]

Tagging @BradLarson @dan-zheng

[asl]

@swift-ci please test

[asl]

@swift-ci please test windows platform

[rxwei]

Should the changes to begin_apply and end_apply be in their own PR before the differentiation aspects?

[asl]

Should the changes to begin_apply and end_apply be in their own PR before the differentiation aspects?

They are and this is #71645. This PR will be rebased after #71645 will be merged.

To be reviewed are 3aba19b and everything after

[asl]

Another pre-requisite is partial_apply patch for coroutines. It is submitted in #71653 As I mentioned, w/o #71653 we will generate proper SIL, but will be unable to proceed further down to real code generation and execution

[asl]

@rxwei Rebased and ready for review. Please take a look when you will have a chance.

[asl]

@swift-ci please test

[asl]

@swift-ci please test

[asl]

@swift-ci please test

[asl]

@swift-ci please test macos platform

[asl]

@swift-ci please test

[asl]

@swift-ci please test macos platform

[asl]

@swift-ci please test macos platform

[asl]

@swift-ci please test

[asl]

@swift-ci please test

[asl]

@swift-ci please test

[asl]

@swift-ci please test macos platform