Ninja: add properties for Swift partial module and doc

When building a swift object, we emit a partial swiftmodule and swiftdoc that must be merged at the end. However, in order to do that, we need to enumerate the swiftmodules and swiftdocs. As a result, the path must be known to CMake. Rather than hardcoding the rules into CMake, create a source property that we can query. This will allow us to create a final placeholder to emit the merge rule.

Issue: #18800 (closed)
Topic-rename: ninja-swift-partial