swift: 4.1.3 -> 4.2.1

* 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

pkgs/development/compilers/swift/patches/0001-build-presets-linux-don-t-require-using-Ninja.patch

@@ -11,7 +11,7 @@ diff --git a/utils/build-presets.ini b/utils/build-presets.ini
 index 7ee57ad2df..e6b0af3581 100644
 --- a/utils/build-presets.ini
 +++ b/utils/build-presets.ini
-@@ -686,7 +686,7 @@ swiftpm
+@@ -717,7 +717,7 @@ swiftpm
  xctest
  dash-dash