Place `.swiftmodule` files a subdirectory named after the configuration.
Fixes: #25864Fixes: #25997
- Swift/RunCMakeTest.cmake:
- CMP0157-OLD was enabled for Xcode, where it works.
- A test was added that verifies .swiftmodule's are generated into
separate directories with multi-config generators.
- Tests/SwiftOnly/CMakeLists.txt: tests were added that validate that
cross-subdirectory module dependencies (via target_link_libraries)
work.