Update ScanDependencies/cxx-overlay-source-lookup.swift (NFC)

This one `CHECK-NEXT: cxx-overlay-source-lookup.swift` used to match
because the expansion of the `%t` temp dir contained the substring
"cxx-overlay-source-lookup.swift", but this is now sanitized to
`TMP_DIR`. The actual source file name is `client.swift`, created by
`split-file` inside the `%t` directory.

Author: hnrklssn

test/ScanDependencies/cxx-overlay-source-lookup.swift

@@ -8,7 +8,7 @@
 /// --------Main module
 // CHECK-LABEL: "modulePath": "deps.swiftmodule",
 // CHECK-NEXT: "sourceFiles": [
-// CHECK-NEXT: cxx-overlay-source-lookup.swift
+// CHECK-NEXT: client.swift
 // CHECK-NEXT: ],
 // CHECK: "directDependencies": [
 // CHECK-DAG: "swift": "Swift"