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 CxxStdlib 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/std/CMakeLists.txt

@@ -52,6 +52,11 @@ add_swift_target_library(swiftCxxStdlib STATIC NO_LINK_NAME IS_STDLIB IS_SWIFT_O
     -Xfrontend -module-interface-preserve-types-as-written
     -enable-experimental-feature AssumeResilientCxxTypes
 
+    # The varying modularization of the C++ standard library on different
+    # platforms makes it difficult to enable MemberImportVisibility for this
+    # module
+    -disable-upcoming-feature MemberImportVisibility
+
     SWIFT_COMPILE_FLAGS_LINUX
     ${SWIFT_SDK_LINUX_CXX_OVERLAY_SWIFT_COMPILE_FLAGS}