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

[Alejandro Colomar via cfe-commits]

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@@ -102,6 +126,11 @@ void test_unspecified_array_length() {
   static_assert(_Countof(**x) == 3);
 }
 
+void test_completed_array() {
+  int a[] = {1, 2, errno};
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alejandro-colomar wrote:

I wanted something that wasn't an ICE, and which the compiler wasn't allowed to transform into an ICE through optimization, to make sure that even with a run-time value, we have _Countof result in an ICE.

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