[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}}
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AaronBallman wrote:

This is going to cause problems when we update to `-std=c23`, so it might be good to guard this on `__STDC_VERSION__` to save ourselves some headaches later.

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