[clang] [clang] support Wold-style-declaration (PR #78837)

[Aaron Ballman via cfe-commits]

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@@ -0,0 +1,33 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wold-style-declaration %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wextra %s
+
+static int x0;
+int __attribute__ ((aligned (16))) static x1; // expected-warning {{'static' is not at beginning of declaration}}
+
+extern int x2;
+int extern x3; // expected-warning {{'extern' is not at beginning of declaration}}
+
+typedef int x4;
+int typedef x5; // expected-warning {{'typedef' is not at beginning of declaration}}
+
+void g (int);
+
+void
+f (void)
+{
+  auto int x6 = 0;
+  int auto x7 = 0; // expected-warning {{'auto' is not at beginning of declaration}}
+  register int x8 = 0;
+  int register x9 = 0; // expected-warning {{'register' is not at beginning of declaration}}
+  g (x6 + x7 + x8 + x9);
+}
+
+const static int x10; // expected-warning {{'static' is not at beginning of declaration}}
+
+/* Attributes are OK before storage class specifiers, since some
+   attributes are like such specifiers themselves.  */
+
+__attribute__((format(printf, 1, 2))) static void h (const char *, ...);
+__attribute__((format(printf, 1, 2))) void static i (const char *, ...); // expected-warning {{'static' is not at beginning of declaration}}
+
+static __thread int var = 5; // not-expected-warning {{'__thread' is not at beginning of declaration}}
----------------
AaronBallman wrote:

You should add tests for: `_Noreturn`, `inline`, `_Thread_local`, `constexpr` (C23), `auto` (C23 usage, not C89 usage).

I think it also makes sense to add a test for `alignas` but I think we should allow that in any position under the same logic for allowing attributes.

Also, please add a newline to the end of the file.

https://github.com/llvm/llvm-project/pull/78837