[wasm] Import wasi_emulated_mman with @_implementationOnly

The `wasi_emulated_mman` module must be imported with `@_implementationOnly`
to avoid requiring Foundation users to enable `_WASI_EMULATED_MMAN` by
themselves. "private import" does not work here because it still imports
private dependencies.

Author: kateinoigakukun

Sources/Foundation/Data.swift

@@ -36,11 +36,11 @@
 @usableFromInline let memcpy = Musl.memcpy
 @usableFromInline let memcmp = Musl.memcmp
 #elseif canImport(WASILibc)
-#if swift(>=6.0)
-private import wasi_emulated_mman
-#else
-import wasi_emulated_mman
-#endif
+// `wasi_emulated_mman` must be imported with `@_implementationOnly` to avoid
+// requiring Foundation users to enable `_WASI_EMULATED_MMAN` by themselves.
+// "private import" does not work here because it still imports private
+// dependencies.
+@_implementationOnly import wasi_emulated_mman
 @usableFromInline let calloc = WASILibc.calloc
 @usableFromInline let malloc = WASILibc.malloc
 @usableFromInline let free = WASILibc.free