Fix support for trailing commas in tuples within generic arguments

[calda]

This PR fixes support for trailing commas in tuples within generic arguments.

This currently fails to compile in Swift 6.2 due to being parsed incorrectly. The type lookahead returns false so it confuses the < for a less-than operator.

// error: binary operator '<' cannot be applied to operands of type 'Array<_>.Type' and '(bar: String, baaz: String).Type'
let _ = Array<(
  bar: String,
  baaz: String,
)>()

Corresponding compiler PR: swiftlang/swift#84421

[calda]

Had to include the substructure: because this test case succeeded as-is without the fix, even though the parsed output was incorrect:

assertParse(
  """
  let _ = Array<(
    bar: String,
    baaz: String,
  )>()
  """
)