[clang] [clang] Disable missing definition warning on pure virtual functions (PR #74510)

[via cfe-commits]

================
@@ -4952,7 +4952,8 @@ void Sema::InstantiateFunctionDefinition(SourceLocation PointOfInstantiation,
         std::make_pair(Function, PointOfInstantiation));
     } else if (TSK == TSK_ImplicitInstantiation) {
       if (AtEndOfTU && !getDiagnostics().hasErrorOccurred() &&
-          !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc())) {
+          !getSourceManager().isInSystemHeader(PatternDecl->getBeginLoc()) &&
+          !Function->isVirtualAsWritten() && !Function->isPure()) {
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cor3ntin wrote:

```suggestion
          !(Function->isVirtualAsWritten() && Function->isPure()) {
```

Can you add a test to make sure non-pure virtual function still get diagnosed?

https://github.com/llvm/llvm-project/pull/74510