Fix a crash when a protocol with a default implementation is aliased with the same name

[d-ronnqvist]

Bug/issue #, if applicable: rdar://107623749

Summary

Fix a crash (from a fatalError in DocumentationContext) when a protocol with a default implementation in one module is both @_export imported and type aliased in another module.

Dependencies

None

Testing

• Create a project two targets—for example named "Outer" and "Inner"—where one target depend on the other
• In the "Inner" target, define a protocol with some requirement and provide a default implementation for that requirement

  // Inner.swift
   public protocol Something {
       func doSomething()
   }
   public extension Something {
       func doSomething() {}
   }
  

• In the "Outer" target, use @_exported import to bring the symbols of the "Inner" target into the "Outer" target and also create a typealias with the same name as protocol from the "Inner" target

  // Outer.swift
  @_exported import Inner
  public typealias Something = Inner.Something
  

• Build documentation for the outer target. This shouldn't crash. Both the protocol requirement and the default implementation should exist under the Something protocol in the Outer target's documentation—like they do in the Inner target's documentation.

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran the ./bin/test script and it succeeded
• Updated documentation if necessary

[d-ronnqvist]

@swift-ci please test

[d-ronnqvist]

@swift-ci please test