Allow type variables to appear inside opened existential archetypes

[slavapestov]

This FIXME comment from #40925 is now fixed:

  // FIXME: Opened archetypes can't be transformed because the
  // the identity of the archetype has to be preserved. This
  // means that simplifying an opened archetype in the constraint
  // system to replace type variables with fixed types is not
  // yet supported. For now, assert that an opened archetype never
  // contains type variables to catch cases where type variables
  // would be applied to the type-checked AST.
  assert(!existential->hasTypeVariable() &&
         "opened existentials containing type variables cannot be simplified");

Fixes rdar://103216428.
Fixes #60547.

[slavapestov]

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[slavapestov]

@swift-ci Please test source compatibility

[slavapestov]

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