Swift: Use per-config module file locations in multi-config generators

Place .swiftmodule files a subdirectory named after the configuration.

Original Description

The initial commits increase the amount of Swift testing done with the Xcode generator, but are not strictly needed to address the problem. The last two commits add a failing test for the problem and then fix the problem.

Fixes: #25864 (closed)
Topic-rename: swift-multi-config