SILGen: Fix key paths that reference internal private(set) decls from other files.

The setter needs to be given hidden linkage so that other files can still form key paths to it.

test/stdlib/KeyPathMultiFile.swift

@@ -0,0 +1,15 @@
+// RUN: %empty-directory(%t)
+// RUN: cp %s %t/main.swift
+// RUN: %target-build-swift -o %t/a.out %t/main.swift %S/Inputs/KeyPathMultiFile_b.swift
+// RUN: %target-run %t/a.out
+
+import StdlibUnittest
+
+var keyPathMultiFile = TestSuite("key paths across multiple files")
+
+keyPathMultiFile.test("identity across multiple files") {
+  expectEqual(A_x_keypath(), \A.x)
+  expectEqual(A_subscript_0_keypath(), \A.[0])
+}
+
+runAllTests()