[5.9][Parse/Sema/SIL] Implement init accessors feature

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Cherry-pick of #66283

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• Adds a new kind of accessor - init accessor - which could be used to initialize one or more stored properties of the type.

  • Just like getter and setter accessors, init accessor could be used on any computed property.

  • Init accessors can specify a set of properties they initialize and accesses (property that should be initialized already) via declaration attributes - initializes and accesses;

  • Example of init accessor that initializes a stored property:

     public struct Test { 
       private var _a: Int
    
       public var a: Int {
         init(initialValue) initializes(_a) {
           _a = initialValue
         }
    
         get { _a }
         set { _a = newValue }
      }
    }

• Modifies memberwise initializers to take advantage of this new mechanism by replacing one or more stored property parameters with corresponding init accessor(s).

  • struct Test from previous example would get init(a: Int) { self.a = a }.

• Adds a new SIL instruction - assign_or_init; Similar to assign_by_wrapper the "mode" is set by DI based on each use site.

  • New intruction also tracks "assignments" - fields that have been previously initialized and would re-assigned by invocation of init accessor, it enables as to call init accessors multiple times even before all fields have been initializes.

• When used in an initializer body, init accessor (i.e. self.a = a) call becomes either an initialization (if all of the fields haven't been initialized yet), or a call to setter.

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@swift-ci please test

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@swift-ci please test

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@swift-ci please test