Tests: Fix SwiftOnly test on Windows

Add public symbols to SubA and SubB.  The test was broken due to a
broken build graph.  The build graph expects libraries to emit the
`.lib` file.  `link.exe` only emits the implib if the library exports at
least one symbol. Because these two libraries were empty, nothing was
generated.

Fixing by adding a public symbol to both libraries.

Issue: #25573

Tests/SwiftOnly/SubA/SubA.swift

@@ -0,0 +1 @@
+public func hi() { print("hi") }

Tests/SwiftOnly/SubB/SubB.swift

@@ -1 +1,3 @@
 import SubA
+
+public var number = 42