Guard tests and targets that require Swift 5.9

Fixtures/MixedTargets/BasicMixedTargets/Package.swift

@@ -88,12 +88,6 @@ let package = Package(
             dependencies: ["MixedTargetWithCustomModuleMapAndResources"]
         ),
 
-        // MARK: - MixedTargetWithCXX_CXXInteropEnabled
-        .target(
-            name: "MixedTargetWithCXX_CXXInteropEnabled",
-            swiftSettings: [.interoperabilityMode(.Cxx)]
-        ),
-
         // MARK: - MixedTargetWithCXX
         .target(
             name: "MixedTargetWithCXX"
@@ -265,10 +259,25 @@ let package = Package(
          // The below two targets are used for testing the above targets.
          .target(name: "SwiftTarget"),
          .target(name: "ClangTarget")
-
-    ],
+    ] + targetsWithCxxInteropEnabled(),
     // TODO(ncooke3): Is the below note behavior that we want to be intended?
     // This is needed for targets with that have
     // `swiftSettings: [.interoperabilityMode(.Cxx)]` set.
     cxxLanguageStandard: .cxx11
 )
+
+func targetsWithCxxInteropEnabled() -> [Target] {
+// The below targets have C++ interoperability mode enabled, a feature that
+// requires Swift 5.9 or greater.
+#if swift(>=5.9)
+  // MARK: - MixedTargetWithCXX_CXXInteropEnabled
+  return [
+    .target(
+      name: "MixedTargetWithCXX_CXXInteropEnabled",
+      swiftSettings: [.interoperabilityMode(.Cxx)]
+    )
+  ]
+#else
+  return []
+#endif  // swift(>=5.9)
+}

Tests/FunctionalTests/MixedTargetTests.swift

@@ -138,6 +138,9 @@ final class MixedTargetTests: XCTestCase {
         } 
     }
 
+// The below tests build targets with C++ interoperability mode enabled, a
+// feature that requires Swift 5.9 or greater.
+#if swift(>=5.9)
     func testMixedTargetWithCXX_CXXInteropEnabled() throws {
         try fixture(name: "MixedTargets/BasicMixedTargets") { fixturePath in
             XCTAssertBuilds(
@@ -146,6 +149,7 @@ final class MixedTargetTests: XCTestCase {
             )
         }
     }
+#endif  // swift(>=5.9)
 
     func testMixedTargetWithCXX() throws {
         try fixture(name: "MixedTargets/BasicMixedTargets") { fixturePath in