swift234

Bring keyword arguments to subscripts.

Subscript declarations were still encoding the names of index
variables in the subscript type, which unintentionally made them
keyword arguments. Bring subscript declarations into the modern day,
using compound names to encode the subscript argument names, which
provides consistency for the keyword-argument world
<rdar://problem/14462349>. Note that arguments in subscripts default
to not being keyword arguments, which seems like the right default.

We now get keyword arguments for subscripts, so one can overload
subscripts on the names of the indices, and distinguish at the call
site. Under -strict-keyword-arguments, we require strictness here as well.

The IRGen/IDE/SILGen test updates are because the mangling of common
subscripts changed from accidentally having keyword arguments to not
having keyword arguments.

Swift SVN r17393