[clang] [Clang] Implement diagnostics for why is_empty is false (PR #145044)

[Aaron Ballman via cfe-commits]

================
@@ -2285,6 +2286,68 @@ static void DiagnoseNonTriviallyCopyableReason(Sema &SemaRef,
   SemaRef.Diag(D->getLocation(), diag::note_defined_here) << D;
 }
 
+static void DiagnoseIsEmptyReason(Sema &S, SourceLocation Loc,
+                                  const CXXRecordDecl *D) {
+  // Non-static data members (ignore zero-width bit‐fields).
+  for (auto *Field : D->fields()) {
+    if (Field->isBitField() && Field->getBitWidthValue() == 0)
----------------
AaronBallman wrote:

We should probably ensure we have tests for dependent values/types, along the lines of:
```
template <int N>
struct S {
  int field : N;
};

S<0> s1; // Empty
S<1> s2; // Non-empty
```

https://github.com/llvm/llvm-project/pull/145044