[clang-tools-extra] [libc++] Prevent calling the projection more than three times (PR #66315)

[Louis Dionne via cfe-commits]

================
@@ -108,7 +108,33 @@ constexpr bool test() {
     auto prvalue_proj = [](const CheckDoubleMove& x) -> CheckDoubleMove { return x; };
     assert(&std::ranges::clamp(val, low, high, moving_comp, prvalue_proj) == &val);
   }
+  { // Make sure we don't call the projection more than three times per [alg.clamp], see #64717
+    int counter = 0;
+    auto projection_function = [counter](const int value) -> int {
----------------
ldionne wrote:

```suggestion
    auto projection_function = [&counter](const int value) -> int {
```

Otherwise the test might not work if we copy the projection itself!

https://github.com/llvm/llvm-project/pull/66315