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

[Shafik Yaghmour via cfe-commits]

================
@@ -0,0 +1,62 @@
+// RUN: %clang_cc1 -std=c2y -verify %s
+
+bool test_if() {
+  if (true) {}
----------------
shafik wrote:

also: https://gcc.godbolt.org/z/9P9Tsb1Yc 

```c
void g() {
  if ( int arr[] = {1,2,3}; arr[1]) {}
}
```

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