Enable 'MemberImportVisibility' upcoming feature in CMake rules (#1022)

Follow-on to #1020 to enable this upcoming feature in the CMake rules,
too.

Resolves rdar://146876960.

- [x] Code and documentation should follow the style of the [Style
Guide](https://github.com/apple/swift-testing/blob/main/Documentation/StyleGuide.md).
- [x] If public symbols are renamed or modified, DocC references should
be updated.

Author: stmontgomery

cmake/modules/shared/CompilerSettings.cmake

@@ -17,7 +17,8 @@ add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-experimental-feature -Xfrontend SuppressedAssociatedTypes>")
 add_compile_options(
   "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend ExistentialAny>"
-  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend InternalImportsByDefault>")
+  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend InternalImportsByDefault>"
+  "SHELL:$<$<COMPILE_LANGUAGE:Swift>:-Xfrontend -enable-upcoming-feature -Xfrontend MemberImportVisibility>")
 
 # Platform-specific definitions.
 if(APPLE)