[clang] [Clang][Sema] Fix err_constexpr_virtual_base diagnostic so that it only diagnoses on constructors and destructors (PR #163690)

[Shafik Yaghmour via cfe-commits]

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@@ -1918,23 +1918,22 @@ static bool CheckConstexprMissingReturn(Sema &SemaRef, const FunctionDecl *Dcl);
 
 bool Sema::CheckConstexprFunctionDefinition(const FunctionDecl *NewFD,
                                             CheckConstexprKind Kind) {
-  const CXXMethodDecl *MD = dyn_cast<CXXMethodDecl>(NewFD);
-  if (MD && MD->isInstance()) {
+  if ((!getLangOpts().CPlusPlus26 && isa<CXXConstructorDecl>(NewFD)) ||
+      ((getLangOpts().CPlusPlus20 && !getLangOpts().CPlusPlus26) &&
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shafik wrote:

We don't have explicit constexpr destructor before C++20 and so this would be a spurious diagnostic.

https://github.com/llvm/llvm-project/pull/163690