Add subtype preserving objective c wrapper to two more locations

[julianlocke]

There were a few more places that the api type subtype information was being thrown away while wrapping for objc compatibility. This PR moves the functions used to create and trigger calling the subtype preserving wrapper to swift_helpers, then instead of writing wrapping code where it is needed we move it to a static method that we can call in the three places we need it.

Continues to fix dropbox/SwiftyDropbox#399

Output code visible in: dropbox/SwiftyDropbox#402

Checklist

General Contributing

• Have you read the Code of Conduct and signed the CLA?

Is This a Code Change?

• Non-code related change (markdown/git settings etc)
• Code Change
• Example/Test Code Change

Validation

• Have you ran tox?
• Do the tests pass?

[codecov]

Codecov Report

Attention: 103 lines in your changes are missing coverage. Please review.

Comparison is base (94fa542) 50.30% compared to head (b7a315d) 49.94%.
Report is 13 commits behind head on swifty-dropbox-beta.

Files	Patch %	Lines
stone/backends/swift_helpers.py	0.00%	34 Missing ⚠️
stone/backends/swift_client.py	0.00%	26 Missing ⚠️
stone/backends/swift_types.py	0.00%	25 Missing ⚠️
stone/cli.py	0.00%	16 Missing ⚠️
stone/backends/swift.py	0.00%	1 Missing ⚠️
stone/frontend/ir_generator.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@                   Coverage Diff                   @@
##           swifty-dropbox-beta     #321      +/-   ##
=======================================================
- Coverage                50.30%   49.94%   -0.37%     
=======================================================
  Files                       37       37              
  Lines                     8740     8803      +63     
  Branches                  1881     1888       +7     
=======================================================
  Hits                      4397     4397              
- Misses                    4026     4089      +63     
  Partials                   317      317              

Flag	Coverage Δ
unit	49.94% <0.96%> (-0.37%)	⬇️

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