[NFC] Stub out llvm::sys::fs::detail::directory_iterator_destruct for GCC 16 (#8559)

That appears to instantiate the llvm::sys::fsdirectory_iterator
destructor from llvm/Support/FileSystem.h even when the type is not
actively used. That destructor calls
llvm::sys::fs::detail::directory_iterator_destruct, which is defined in
Unix/Path.inc, but which is #if'ed out, causing builds to fail with

> [ 87%] Linking CXX shared library lib/libbinaryen.so
> /usr/bin/ld.bfd:
third_party/llvm-project/CMakeFiles/llvm_dwarf.dir/DWARFUnit.cpp.o: in
function
`std::_Sp_counted_base<(__gnu_cxx::_Lock_policy)2>::_M_release() [clone
.part.0]':
> DWARFUnit.cpp:(.text+0x2665): undefined reference to
`llvm::sys::fs::detail::directory_iterator_destruct(llvm::sys::fs::detail::DirIterState&)'
> /usr/bin/ld.bfd: DWARFUnit.cpp:(.text+0x2855): undefined reference to
`llvm::sys::fs::detail::directory_iterator_destruct(llvm::sys::fs::detail::DirIterState&)'
> /usr/bin/ld.bfd:
third_party/llvm-project/CMakeFiles/llvm_dwarf.dir/DWARFUnit.cpp.o: in
function `llvm::DWARFUnit::~DWARFUnit()':
> DWARFUnit.cpp:(.text+0x2c13): undefined reference to
`llvm::sys::fs::detail::directory_iterator_destruct(llvm::sys::fs::detail::DirIterState&)'
> /usr/bin/ld.bfd: DWARFUnit.cpp:(.text+0x2e2c): undefined reference to
`llvm::sys::fs::detail::directory_iterator_destruct(llvm::sys::fs::detail::DirIterState&)'
> /usr/bin/ld.bfd: DWARFUnit.cpp:(.text+0x2ff2): undefined reference to
`llvm::sys::fs::detail::directory_iterator_destruct(llvm::sys::fs::detail::DirIterState&)'
> /usr/bin/ld.bfd:
third_party/llvm-project/CMakeFiles/llvm_dwarf.dir/DWARFUnit.cpp.o:DWARFUnit.cpp:(.text+0x4d39):
more undefined references to
`llvm::sys::fs::detail::directory_iterator_destruct(llvm::sys::fs::detail::DirIterState&)'
follow

Provide a trivial stub to make the build succeed.

Author: stbergmann

third_party/llvm-project/Path.cpp

@@ -1079,6 +1079,14 @@ ErrorOr<perms> getPermissions(const Twine &Path) {
 #if defined(_WIN32)
 #include "Windows/Path.inc"
 #endif
+#else
+// Stub out directory_iterator_destruct, which at least GCC 16 instantiates from
+// llvm/Support/FileSystem.h even though binaryen never uses it:
+namespace llvm { namespace sys { namespace fs { namespace detail {
+std::error_code directory_iterator_destruct(DirIterState &) {
+  return std::error_code();
+}
+}}}}
 #endif
 
 #if 0 // XXX BINARYEN