[clang] [Clang][RFC] Intrododuce a builtin to determine the structure binding size (PR #131515)

[Eric Niebler via cfe-commits]

================
@@ -434,6 +434,36 @@ __datasizeof
 ``__datasizeof`` behaves like ``sizeof``, except that it returns the size of the
 type ignoring tail padding.
 
+.. _builtin_structured_binding_size-doc:
+
+__builtin_structured_binding_size (C++)
+---------------------------------------
+``__builtin_structured_binding_size`` returns the *structured binding size*
+([dcl.struct.bind]) of the type ``T`` (or unevaluate expression ``arg``)
----------------
ericniebler wrote:

```suggestion
([dcl.struct.bind]) of the type ``T`` (or unevaluated expression ``arg``)
```


https://github.com/llvm/llvm-project/pull/131515