[clang] [Clang] Fixed a crash when instantiating an invalid out-of-line static data member definition in a local class (PR #196772)

[via cfe-commits]

================
@@ -0,0 +1,12 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s
+
+template <class T> int f(T) {
+  struct A {
+    static int B; // expected-error {{static data member 'B' not allowed in local struct 'A'}}
+  };
+  int A::B; // expected-note {{previous definition is here}}
+  int A::B = 1; // expected-error {{redefinition of 'B'}}
+  return 0;
+}
+
+int x = f(0);
----------------
TPPPP72 wrote:

In fact, this is not just a feeling, but a proven fact.

In `VisitVarDecl`, I added these:
```cpp
if (D->getIdentifier()) {
  llvm::errs() << "Checking VarDecl: " << D->getNameAsString() << "\n";
  llvm::errs() << "  D->isInvalidDecl(): " << D->isInvalidDecl() << "\n";
  llvm::errs() << "  Var->isInvalidDecl(): " << Var->isInvalidDecl() << "\n";
  llvm::errs() << "  Var Context Kind: " << Var->getDeclContext()->getDeclKindName() << "\n";
  if (auto *FD = dyn_cast<FunctionDecl>(Var->getDeclContext())) {
      llvm::errs() << "  Var Context Name: " << FD->getNameAsString() << "\n";
  }
}
```

```
Checking VarDecl: B
  D->isInvalidDecl(): 0
  Var->isInvalidDecl(): 0
  Var Context Kind: Function
  Var Context Name: f
```

I'm not saying the entire node is valid, but rather that it's valid specifically for B. Whether B's state is valid can only be determined by parsing the instantiation of A.

https://github.com/llvm/llvm-project/pull/196772