[cxx-interop] Add tests for new interop flag/diagnostics; fix a diagnostic.

Author: zoecarver

include/swift/AST/DiagnosticsFrontend.def

@@ -520,7 +520,7 @@ ERROR(dont_enable_interop_and_compat,none,
 
 ERROR(invalid_interop_compat_mode,none,
       "invalid option passed to -cxx-interoperability-mode. Please select either "
-      "'off' or 'swift-5.7'.", ())
+      "'off' or 'swift-5.9'.", ())
 NOTE(swift_will_maintain_compat,none,
      "Swift will maintain source compatibility for imported APIs based on the "
      "selected compatibility mode, so updating the Swift compiler will not "

test/Interop/Cxx/driver/dont-enable-interop-compat.swift

@@ -0,0 +1,19 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: %target-swift-frontend -typecheck -I %t/Inputs  %t/test.swift  -enable-experimental-cxx-interop 2>&1 | %FileCheck %s
+
+//--- Inputs/module.modulemap
+module Test {
+    header "test.h"
+    requires cplusplus
+}
+
+//--- Inputs/test.h
+
+
+//--- test.swift
+
+import Test
+
+// CHECK: warning: the -enable-experimental-cxx-interop flag is deprecated. Please pass '-cxx-interoperability-mode=' instead.
+// CHECK: note: Swift will maintain source compatibility for imported APIs based on the selected compatibility mode, so updating the Swift compiler will not change how APIs are imported.

test/Interop/Cxx/driver/enable-interop-flag-depr.swift

@@ -0,0 +1,18 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: not %target-swift-frontend -typecheck -I %t/Inputs  %t/test.swift  -enable-experimental-cxx-interop -cxx-interoperability-mode=swift-5.9 2>&1 | %FileCheck %s
+
+//--- Inputs/module.modulemap
+module Test {
+    header "test.h"
+    requires cplusplus
+}
+
+//--- Inputs/test.h
+
+//--- test.swift
+
+import Test
+
+// CHECK: error: do not pass both -enable-experimental-cxx-interop and -cxx-interoperability-mode. Remove -enable-experimental-cxx-interop.
+// CHECK: note: Swift will maintain source compatibility for imported APIs based on the selected compatibility mode, so updating the Swift compiler will not change how APIs are imported.

test/Interop/Cxx/driver/invalid-interop-compat-mode.swift

@@ -0,0 +1,18 @@
+// RUN: rm -rf %t
+// RUN: split-file %s %t
+// RUN: not %target-swift-frontend -typecheck -I %t/Inputs  %t/test.swift  -cxx-interoperability-mode=swift-5.8 2>&1 | %FileCheck %s
+
+//--- Inputs/module.modulemap
+module Test {
+    header "test.h"
+    requires cplusplus
+}
+
+//--- Inputs/test.h
+
+//--- test.swift
+
+import Test
+
+// CHECK: error: invalid option passed to -cxx-interoperability-mode. Please select either 'off' or 'swift-5.9'.
+// CHECK: note: Swift will maintain source compatibility for imported APIs based on the selected compatibility mode, so updating the Swift compiler will not change how APIs are imported.