SE-0209: support both swift 4.2 and 5

Package.swift

@@ -25,5 +25,6 @@ let package = Package(
                 "Performance",
                 "SPM"
             ])
-    ]
+    ],
+    swiftLanguageVersions: [.v4_2, .version("5")]
 )

Tests/SPM/Sources/SPM/main.swift

@@ -8,3 +8,14 @@ let sqlVersion = try! DatabaseQueue().read { db in
     try String.fetchOne(db, sql: "SELECT sqlite_version()")!
 }
 print("SQLite version from SQL function: \(sqlVersion)")
+
+#if swift(>=5.0)
+try! DatabaseQueue().write { db in
+    try db.execute(literal: """
+        CREATE TABLE t(a);
+        INSERT INTO t VALUES(\("5"));
+        """)
+    let swiftVersion = String.fetchOne(db, sql: "SELECT a FROM t")!
+    print("Swift version from SQL: \(swiftVersion)")
+}
+#endif