[clang] [C] Add -Wduplicate-decl-specifier to -Wc++-compat (PR #138012)

[Shafik Yaghmour via cfe-commits]

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@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -fsyntax-only -verify -Wduplicate-decl-specifier %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wc++-compat %s
+// RUN: %clang_cc1 -fsyntax-only -verify -Wno-duplicate-decl-specifier -Wc++-compat %s
+// RUN: %clang_cc1 -fsyntax-only -verify=good -Wc++-compat -Wno-duplicate-decl-specifier %s
+// RUN: %clang_cc1 -fsyntax-only -verify=good -Wno-duplicate-decl-specifier %s
+// RUN: %clang_cc1 -fsyntax-only -verify -x c++ %s
+// good-no-diagnostics
+
+// Note: we treat this as a warning in C++, so you get the same diagnostics in
+// either language mode. However, GCC diagnoses this as an error, so the
+// compatibility warning has value.
+const const int i = 12; // expected-warning {{duplicate 'const' declaration specifier}}
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shafik wrote:

`volatile const volatile const int x = 1;` might also be worth checking, to get in `volatile` and test that we catch alternating qualifies.

https://github.com/llvm/llvm-project/pull/138012