[llvm] [llvm] replace static_assert with std::enable_if_t in ilist_node_impl (PR #127722)

[Andrew Rogers via llvm-commits]

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@@ -147,9 +149,8 @@ class ilist_node_impl
   ///
   /// This requires sentinel tracking to be explicitly enabled.  Use the
   /// ilist_sentinel_tracking<true> option to get this API.
-  bool isSentinel() const {
-    static_assert(OptionsT::is_sentinel_tracking_explicit,
-                  "Use ilist_sentinel_tracking<true> to enable isSentinel()");
+  template <typename T = OptionsT>
+  std::enable_if_t<T::is_sentinel_tracking_explicit, bool> isSentinel() const {
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andrurogerz wrote:

Right, thanks @compnerd.

This change is not intended to be temporary nor is it working around a compiler bug. If you build llvm main on Windows right now with `LLVM_BUILD_LLVM_DYLIB=ON` and `LLVM_BUILD_LLVM_DYLIB_VIS=ON`, compilation will fail due to the `static_assert`.

This change fixes the bug by enabling the `ilist_node_impl` class to be fully instantiated when configured with `is_sentinel_tracking_explicit=false`, which is required to DLLExport it. It also maintains the strictness enforced by the `static_assert` by failing compilation if someone tries to call `isSentinel()` on a node configured without explicit sentinel tracking.

An alternative fix is to conditionally compile the `static_assert` like so (tbh, this was my original fix):
```C++
  bool isSentinel() const {
#if !defined(_WIN32) || !defined(LLVM_BUILD_LLVM_DYLIB)
    static_assert(OptionsT::is_sentinel_tracking_explicit,
                  "Use ilist_sentinel_tracking<true> to enable isSentinel()");
#endif 
    return node_base_type::isSentinel();
  }
```
That version might be a little easier to understand, but it is messier and less strict so I prefer the `enable_if` version.

https://github.com/llvm/llvm-project/pull/127722