swift234

Eliminate default arguments from TupleType.

In Swift, default arguments are associated with a function or
initializer's declaration---not with its type. This was not always the
case, and TupleType's ability to store a default argument kind is a
messy holdover from those dark times.

Eliminate the default argument kind from TupleType, which involves
migrating a few more clients over to declaration-centric handling of
default arguments. Doing so is usually a bug-fix anyway: without the
declaration, one didn't really have

The SILGen test changes are due to a name-mangling fix that fell out
of this change: a tuple type is mangled differently than a non-tuple
type, and having a default argument would make the parameter list of a
single-parameter function into a tuple type. Hence,

  func foo(x: Int = 5)

would get a different mangling from

  func foo(x: Int)

even though we didn't actually allow overloading.

Fixes rdar://problem/24016341, and helps us along the way to SE-0111
(removing the significance of argument labels) because argument labels
are also declaration-centric, and need the same information.