WITOverlayGenerator: Fix build with Swift 6.1

The `Extern` feature was accidentally added as an implicitly available
feature in Swift 6.0, but it should have been an experimental feature.

And importing with `@_implementationOnly` without library evolution
is now warned about, so we need to import the shims module privately.

Author: kateinoigakukun

Sources/WITOverlayGenerator/GuestGenerators/GuestPrelude.swift

@@ -7,7 +7,11 @@ let guestPrelude = """
     #if canImport(WASILibc)
     import WASILibc
     #endif
+    #if compiler(>=6.0)
+    private import _CabiShims
+    #else
     @_implementationOnly import _CabiShims
+    #endif
 
     fileprivate enum Prelude {
         class LeakBox<Wrapped> {

Tests/WITOverlayGeneratorTests/Runtime/RuntimeTestHarness.swift

@@ -165,6 +165,7 @@ struct RuntimeTestHarness {
             arguments: [
                 "-target", "wasm32-unknown-none-wasm",
                 "-enable-experimental-feature", "Embedded",
+                "-enable-experimental-feature", "Extern",
                 "-wmo", "-Xcc", "-fdeclspec",
                 "-Xfrontend", "-disable-stack-protector",
                 "-Xlinker", "--no-entry", "-Xclang-linker", "-nostdlib",
@@ -176,6 +177,7 @@ struct RuntimeTestHarness {
             inputFiles: inputFiles,
             arguments: [
                 "-target", "wasm32-unknown-wasi",
+                "-enable-experimental-feature", "Extern",
                 "-static-stdlib",
                 "-Xclang-linker", "-mexec-model=reactor",
                 "-resource-dir", configuration.wasiSwiftSDKPath.appendingPathComponent("/swift.xctoolchain/usr/lib/swift_static").path,