update Swift
* new tweaks to the build process are mostly documented inline
* the swiftc wrapper needs the linker path for libicu
* some older tweaks could be removed
* remove fuzz from patches by updating line numbers
* one build fix to libdispatch has been filed upstream
* necessary tweaks and patches to get swift 4.2.1 to build
* disabled more tests because they would require additional patching
* delete empty directories from final build output
* some minor cleanup
