Use ObjC reserved words only when objcCompatible is enabled

[wing328]

PR checklist

• Read the contribution guidelines.
• Ran the shell script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh, ./bin/openapi3/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates). Windows batch files can be found in .\bin\windows\.
• Filed the PR against the correct branch: master, 3.4.x, 4.0.x. Default: master.
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Description of the PR

To fix #2589

cc @jgavris (2017/07) @ehyche (2017/08) @Edubits (2017/09) @jaz-ah (2017/09) @d-date (2018/03)

[d-date]

👍

[wing328]

Upgrade Note

The following ObjC reserved words will only be included in the reserved word list when the "objcCompatible" option is enabled:

"id", "description", "NSArray", "NSURL", "CGFloat", "NSSet", "NSString", "NSInteger"
"NSUInteger", "NSError", "NSDictionary"

For example, _id will become just id for a Swift only client.