Rather than parsing the call arguments (or similar, e.g., subscript)
as a parenthesized expression or tuple, then later reworking that
ParenExpr/TupleExpr if a trailing closure comes along, then digging
through that ParenExpr/TupleExpr to pull out the arguments and
trailing closure... just parse the expression list and trailing
closure together, then directly form the appropriate AST node with
arguments/labels/label locations/trailing closure.
Fixes rdar://problem/19804707, which is an issue where trailing
closures weren't working with unresolved member expressions (e.g.,
".foo {... }"), and is a stepping-stone to SE-0111.
