wasm: Enable filesystem APIs in libcxx

As of a recent fix included in LLVM 17[1] and wasi-libc fix[2], we can
enable `LIBCXX_ENABLE_FILESYSTEM` in libcxx build for WebAssembly/WASI.
This allows us to use `<filesystem>`, `<fstream>`, etc in C++ code.

[1]: llvm/llvm-project@66a562d
[2]: WebAssembly/wasi-libc#463

utils/swift_build_support/swift_build_support/products/wasisysroot.py

@@ -203,7 +203,7 @@ def _build(self, host_target, enable_wasi_threads=False,
         self.cmake_options.define('LIBCXX_ENABLE_SHARED:BOOL', 'FALSE')
         self.cmake_options.define('LIBCXX_ENABLE_EXPERIMENTAL_LIBRARY:BOOL', 'FALSE')
         self.cmake_options.define('LIBCXX_ENABLE_EXCEPTIONS:BOOL', 'FALSE')
-        self.cmake_options.define('LIBCXX_ENABLE_FILESYSTEM:BOOL', 'FALSE')
+        self.cmake_options.define('LIBCXX_ENABLE_FILESYSTEM:BOOL', 'TRUE')
         self.cmake_options.define('LIBCXX_CXX_ABI', 'libcxxabi')
         self.cmake_options.define('LIBCXX_HAS_MUSL_LIBC:BOOL', 'TRUE')