[Foundation] Correct symbolic availability for CFURL component characer set accessors

[phausler]

This resolves rdar://problem/32188090
tvOS 9.0 did not have the symbols for the CFURLComponents accessor methods for character sets; this change works around that to use the Foundation variants via shim functions

[phausler]

@swift-ci please test

[swift-ci]

Build failed
Jenkins build - Swift Test OS X Platform
Git Commit - f077698
Test requested by - @phausler

[phausler]

/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/swift/test/Constraints/disjunction_ordering_failures.swift:8:11: error: unexpected error produced: use of undeclared type 'CGFloat'
17:22:47 extension CGFloat: P { }
17:22:47           ^
17:22:47 /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-master/swift/test/Constraints/disjunction_ordering_failures.swift:11:10: error: unexpected error produced: use of undeclared type 'CGFloat'
17:22:47   let h: CGFloat = 0

Pretty certain CharacterSet should not have interaction with CGFloat

[phausler]

@swift-ci please test