[libc++] Implement `operator==` for `filesystem::space_info`

Implements part of P1614R2 "The Mothership has Landed"

Differential Revision: https://reviews.llvm.org/D130861

NOKEYCHECK=True
GitOrigin-RevId: 706b3951b38961ffa6581d446fee6664038caced
diff --git a/include/filesystem b/include/filesystem
index 87de886..aa0cd56 100644
--- a/include/filesystem
+++ b/include/filesystem
@@ -64,6 +64,8 @@
       uintmax_t capacity;
       uintmax_t free;
       uintmax_t available;
+
+      friend bool operator==(const space_info&, const space_info&) = default; // C++20
     };
 
     enum class file_type;