[clang] [Clang] Fix a crash introduced in PR#88666 (PR #89567)

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@@ -0,0 +1,7 @@
+// RUN: %clang_cc1 -x c++ -verify %s
+// expected-no-diagnostics
+
+template <int Unroll> void foo() {
+  #pragma unroll Unroll
+  for (int i = 0; i < Unroll; ++i);
+}
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yronglin wrote:

I'd like to add a test for '#pragma GCC unroll N', WDYT?

https://github.com/llvm/llvm-project/pull/89567