Fix warnings in the Foundation build

[pushkarnk]

Foundation/HTTPCookieStorage.swift:80:29: warning: 'init(contentsOfFile:)' is deprecated
        guard let cookies = NSMutableDictionary(contentsOfFile: cookieFilePath) else { return }

and

Foundation/Data.swift:1593:13: warning: initialization of immutable value 'length' was never used; consider replacing with assignment to '_' or removing it
        let length = count

[alblue]

@swift-ci please test

[pushkarnk]

@swift-ci please test

[alblue]

This seems to have unrelated failures in SourceKit:

/home/buildnode/jenkins/workspace/swift-corelibs-foundation-PR-Linux/swift/test/SourceKit/Misc/stats.swift:36:12: error: expected string not found in input
// SEMA_2: 0 {{.*}} source.statistic.num-ast-cache-hits

[alblue]

@swift-ci please clean test

[pushkarnk]

@swift-ci please clean test

[alblue]

@swift-ci please smoke test

[alblue]

@shahmishal is there an issue with swift-ci not picking up triggers for the Linux platform at the moment?

[shahmishal]

@swift-ci please smoke test

[shahmishal]

@swift-ci please test

[shahmishal]

Foundation repo does not support smoke test trigger, just full test.

[alblue]

@shahmishal ah, ok - presumably it doesn't support the clean test either?

[alblue]

LGTM

@swift-ci please test and merge

[alblue]

@swift-ci please test and merge