Swift: Add CMAKE_LINKER_TYPE support

Evan Wilderequested to merge
etcwilde/cmake:ewilde/swift-linker-selection into master

Extend !8861 (merged) with the CMAKE_Swift_USING_LINKER_* variables needed to support CMAKE_LINKER_TYPE for Swift.

Backport: release
Topic-rename: swift-linker-selection

Edited by Brad King

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