[clang] [clang][ExprConst] Fix second arg of __builtin_{clzg,ctzg} not always being evaluated (PR #86742)

[via cfe-commits]

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@@ -0,0 +1,39 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+// expected-no-diagnostics
+
+constexpr int increment(int& x) {
+  x++;
+  return x;
+}
+
+constexpr int test_clzg_0() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_clzg(0U, increment(x));
+  return x;
+}
+
+static_assert(test_clzg_0() == 1);
+
+constexpr int test_clzg_1() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_clzg(1U, increment(x));
+  return x;
+}
+
+static_assert(test_clzg_1() == 1);
+
+constexpr int test_ctzg_0() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_ctzg(0U, increment(x));
+  return x;
+}
+
+static_assert(test_ctzg_0() == 1);
+
+constexpr int test_ctzg_1() {
+  int x = 0;
+  [[maybe_unused]] int unused = __builtin_ctzg(1U, increment(x));
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overmighty wrote:

I guess using a C-style cast is fine if it's to cast to `void`.

Adding `-Wno-unused-value` sounds a bit excessive however.

https://github.com/llvm/llvm-project/pull/86742