Add AST representation for coroutines

[asl]

This is a proof-of-concept PR adding AST representation for coroutines

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

[asl]

@tkremenek @rjmccall Following discussions on LLVM Dev Meeting, this is a stripped-down version of #78508:

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

Essentially the PR contains AST-related bits for coroutine declarations and coroutine function types as well as required boilerplate for AST mangling, dumping, serialization, etc.

@xedin comments from the original PR were addressed as well.

[asl]

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