relates to:

• SR-5441 Closure Capture ABI

Issue Description:

Once the Closure Capture ABI is implemented (https://bugs.swift.org/browse/SR-5441), there are some followup changes I would like to make to the SIL representation:

• The SIL level partial_apply could take a single box for its context instead of one
  for each captured variable. All it does is bind the context to the function.

• Unify SIL closure argument types: Let's do away with the strange
  SILGen "optimization" that sometimes converts nonescaping closure
  arguments to to @inout_aliasable. I don't think this accomplishes
  anything useful today but adds a lot of complexity.

• Reliably distinguish noescaping from escaping SIL closure arguments
  as part of the SIL convention. This gives us a nice invariant that
  addresses are always statically exclusive. Nonescaping closures
  would always take arguments by address ($T*, $U*), and escaping
  closures would take arguments by box (${ var T }, ${ var U}).