Add AST representation for coroutines

[asl]

Make use of it to differentiate array subscript modify accessor.

[asl]

@swift-ci please test

[asl]

This is a proof-of-concept PR adding AST representation for coroutines and making use of it for array subscript modify accessor differentiation.

Further background at https://forums.swift.org/t/pitch-yield-once-functions-first-class-coroutines/77081

[asl]

swiftlang/swift-syntax#2932

@swift-ci please test

[asl]

@swift-ci please test

[asl]

Rebased applying suggestions from the code review. Also, the commits were rearranged, so first commits in the stack represent common changes. So far the only way to invoke all the code is via autodiff we do not have a Swift way to invoke coroutines, only define them.

[asl]

Please test with following pull request: swiftlang/llvm-project#11396
@swift-ci please test

[asl]

#85372 contains rebased and stripped down version to simplify review:

• No autodiff-related parts
• No silgen (besides the "standard" special path for coroutine accessors)
• Some other fixes for sil optimizer is omitted