stdlib: Enable upcoming MemberImportVisibility feature.

It was already supposed to be enabled, but I forgot to update the CMake when I
renamed the feature from ExtensionImportVisibility to MemberImportVisibility.

However, leave it disabled on the Cxx module since the C++ standard library's
modularization varies a lot by platform, making it difficult to add the right
conditional imports.

Author: tshortli

stdlib/cmake/modules/SwiftSource.cmake

@@ -633,10 +633,8 @@ function(_compile_swift_files
   list(APPEND swift_flags "-enable-experimental-feature" "SuppressedAssociatedTypes")
   list(APPEND swift_flags "-enable-experimental-feature" "SE427NoInferenceOnExtension")
   list(APPEND swift_flags "-enable-experimental-feature" "AllowUnsafeAttribute")
-
   list(APPEND swift_flags "-enable-experimental-feature" "NonescapableTypes")
-
-  list(APPEND swift_flags "-enable-experimental-feature" "ExtensionImportVisiblity")
+  list(APPEND swift_flags "-enable-upcoming-feature" "MemberImportVisibility")
 
   if (SWIFT_STDLIB_ENABLE_STRICT_CONCURRENCY_COMPLETE)
     list(APPEND swift_flags "-strict-concurrency=complete")

stdlib/public/Cxx/CMakeLists.txt

@@ -25,6 +25,10 @@ add_swift_target_library(swiftCxx STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_ONLY
     # For functionality that depends on the C++ stdlib, use C++ stdlib overlay (`swiftstd` module).
     -Xcc -nostdinc++
 
+    # The varying modularization of the C++ standard library on different platforms
+    # makes it difficult to enable MemberImportVisibility on this target
+    -disable-upcoming-feature MemberImportVisibility
+
     DEPLOYMENT_VERSION_OSX ${COMPATIBILITY_MINIMUM_DEPLOYMENT_VERSION_OSX}
 
     LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"