[clang] [Clang][C2y] Add support for if declarations (N3356 paper) (PR #198244)

[Aaron Ballman via cfe-commits]

================
@@ -0,0 +1,198 @@
+// RUN: %clang_cc1 -std=c2y -verify %s
----------------
AaronBallman wrote:

Can you add the usual file header?
```
/* WG14 N3267: Clang 24
 * 'if' declarations, v2
 */
```

https://github.com/llvm/llvm-project/pull/198244