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

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@@ -55,7 +55,7 @@ bool negative_test_if() {
                                         expected-error {{expected expression}}
                                         expected-warning {{expression result unused}}*/
   if (int x) {} // expected-error {{variable declaration in condition must have an initializer}}
-  if ([[]]; true) {} // expected-warning {{empty declaration statement of 'if' has no effect}}
+  if ([[]]; true) {} // expected-error {{first clause in condition must be a declaration}}
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Sirraide wrote:

I believe this should be valid because `[[]];` is a valid _attribute-declaration_, which is a _declaration_ in C (but not in C++...).

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