[6.1][TypeChecker/SILGen] Allow any Sendable to match Any while matching generic arguments

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Cherry-pick of #78171

---

• Explanation:

  It's currently impossible for libraries to adopt any Sendable in some positions
  when the type was Any before.

  For example if library had a member of dictionary type like:

  public struct MyAPI {
     public let myInfo: [String: Any]
  }

  and users added extensions to MyAPI that reflect the fact that Value generic
  parameter is always Any:

  extension Dictionary where Value == Any {
    func computeSomething() { ... }
  }

  It won't be possible for the authors of MyAPI to change value type to any Sendable
  i.e. @preconcurrency public let myInfo: [String: any Sendable],
  while adopting type to Swift concurrency, without breaking existing clients.

  To address this, allow any Sendable to match Any constraint while matching
  generic arguments i.e. [any Sendable] -> [Any] when any Sendable
  type comes from context that involves @preconcurrency declarations.

  To facilitate that we need a new expression that models an unsafe bitcast
  from one type to the other - UnsafeCastExpr. This expression is going
  to be introduced by the solver during solution application phase to
  erase any Sendable to Any in generic argument positions.

• Main Branch PR: [TypeChecker/SILGen] Allow any Sendable to match Any while matching generic arguments #78171

• Resolves: rdar://140300022

• Risk: Low (This is a narrow fix that applies only to generic argument contexts of @preconcurrency declarations and is limited to any Sendable -> Any directional conversions).

• Reviewed By: @hborla @atrick

• Testing: Added new tests to the concurrency, SILGen, and interpreter test suites.

[xedin]

@swift-ci please test

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@swift-ci please test