[clang] Add more tests for _Countof (PR #133333)

[Aaron Ballman via cfe-commits]

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@@ -144,3 +177,13 @@ void test_compound_literals() {
   static_assert(_Countof((int[2]){}) == 2);
   static_assert(_Countof((int[]){1, 2, 3, 4}) == 4);	
 }
+
+static int test_f1();
+static int test_f2();  // expected-warning {{never defined}}
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AaronBallman wrote:

Pretty sure this would be `function 'test_f2' has internal linkage but is not defined`

https://github.com/llvm/llvm-project/pull/133333