Swift: Improvements related to the swift/cleartext-logging query.

[geoffw0]

Improvements related to the swift/cleartext-logging query:

• model flow through getVaList (so that we can pass the NSLogv test cases).
• add data flow through variadic arguments (this will have effects more widely than the swift/cleartext-logging query and is separately tested).
• add an allowImplicitRead for array content sinks in the swift/cleartext-logging query (so that, together with the previous bullet point, we can pass the NSLog and more of the print test cases).
• add more sinks for the swift/cleartext-logging query.