[Parser] Allow parsing any in structural position within enum associated value types.

[hborla]

Previously, this code would result in an error:

protocol P {}

enum E {
  case hello((any P) -> Void) // error: expected parameter name followed by ':'
}

This change simply calls into Parser::startsParameterName, which already supports looking for contextual any and some, instead of repeating the code without checking for contextual keywords in Parser::canParseTypeTupleBody.

Resolves: rdar://93382182

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please test source compatibility