stdlib: Adopt ExtensionImportVisibility experimental feature.

This didn't expose any missing imports in the sources of any of the standard
library modules, but that's not entirely surprising. Still, we should have the
feature enabled to test it and prevent regressions from creeping in before the
behavior becomes the default.

Exclude `ExtensionImportVisibility` from module interfaces.

Author: tshortli

include/swift/Basic/Features.def

@@ -365,7 +365,7 @@ EXPERIMENTAL_FEATURE(ClosureIsolation, true)
 CONDITIONALLY_SUPPRESSIBLE_EXPERIMENTAL_FEATURE(IsolatedAny, true)
 
 // Whether members of extensions respect the enclosing file's imports.
-EXPERIMENTAL_FEATURE(ExtensionImportVisibility, true)
+EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE(ExtensionImportVisibility, true)
 
 // Alias for IsolatedAny
 EXPERIMENTAL_FEATURE(IsolatedAny2, true)

stdlib/cmake/modules/SwiftSource.cmake

@@ -628,6 +628,8 @@ function(_compile_swift_files
     list(APPEND swift_flags "-enable-experimental-feature" "NonescapableTypes")
   endif()
 
+  list(APPEND swift_flags "-enable-experimental-feature" "ExtensionImportVisiblity")
+
   if (SWIFT_STDLIB_ENABLE_STRICT_CONCURRENCY_COMPLETE)
     list(APPEND swift_flags "-strict-concurrency=complete")
   endif()