[Swift3] Fix parameter names using {{baseName}}

[ePaul]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change.
  → bin/swift3-petstore-all.sh (once before the change, see Update swift3 samples after 2.2.2. #4911, once again after the change.)
• Filed the PR against the correct branch: master for non-breaking changes.

Description of the PR

This includes the changes of #4911 – after merging those, it should be clearer what actually changed. (Or just have a look at the last two commits.)

This fixes a wrong name-mapping of path parameters, as part of solving #4898.

It looks like there is no test case in petstore which actually makes a difference here.
It might be that there actually is no problem (i.e. if parameter names are never modified for Swift3, then baseName is the same as paramName, so it doesn't matter – though then there might be problems when hitting reserved words), or that it just is not happens with our petstore example.

I need someone who knows about the swift clients to review this – cc @jaz-ah @Edubits.

(A side remark: Some ids in the generated samples look different each time the generator is run – see commit 8e847ab. This introduces unneeded noise when looking at the diffs. Might be worth looking into, in a separate issue/PR.)

[wing328]

@ePaul thanks for the fix.

cc @hexelon @jaz-ah @Edubits @jgavris

[ePaul]

Rebased onto master (and reran the samples generation) to get rid of conflicts.

[wing328]

LGTM. Thanks for the fix.