[libcxx] [llvm] [libc++] Kickstart C++29 support (PR #215820)

[A. Jiang via llvm-commits]

================
@@ -1533,6 +1533,14 @@ def add_version_header(tc):
             },
             "headers": ["variant"],
         },
+        {
+            "name": "__cpp_lib_view_interface",
+            "values": {
+                "c++29": 202606,
----------------
frederick-vs-ja wrote:

Basically LGTM.

Possibly improvement:

```suggestion
                "c++29": 202606,  # P4179R0 view_interface::[c]rbegin()
```

Also, it might be more consistent to remove this entry or add all new C++29 FTMs.

https://github.com/llvm/llvm-project/pull/215820