Add documentation generation test

Author: MaxDesiatov

test/Documentation/docc.swift

@@ -0,0 +1,6 @@
+// REQUIRES: OS=macosx
+
+// RUN: mkdir $(dirname %t)/out
+// RUN: xcrun docc convert --output-path $(dirname %t)/out --ide-console-output --allow-arbitrary-catalog-directories $(dirname %utils)/docs
+// RUN: test $(du -c $(dirname %t)/out | tail -1 | awk '{ print $1 }') -gt 18000 && echo success
+// CHECK: success