Merge topic 'swift-improvements'

f19c189e74 Merge branch 'backport-swift-improvements' into swift-improvements
5803a8a2f7 Swift: support SONAME on ELFish targets
47e28cbe1f Swift: support multithreaded compilation
6814567e8c Swift: add rules for static linking
c8741e8ff5 Swift: Add library search paths for dependencies
be0d404208 Support per-language library link flags

Acked-by: Kitware Robot <[email protected]>
Merge-request: !3568