[clang] [C++] Fix a crash with __thread and dependent types (PR #140542)

[Aaron Ballman via cfe-commits]

================
@@ -14608,6 +14608,10 @@ void Sema::CheckCompleteVariableDeclaration(VarDecl *var) {
   std::optional<bool> CacheHasConstInit;
   const Expr *CacheCulprit = nullptr;
   auto checkConstInit = [&]() mutable {
+    const Expr *Init = var->getInit();
+    if (Init->isInstantiationDependent())
----------------
AaronBallman wrote:

That's the case I think isn't problematic, because of the predicate:
```
  if ((getLangOpts().CPlusPlus || (getLangOpts().C23 && var->isConstexpr())) &&
      !type->isDependentType() && Init && !Init->isValueDependent() &&
      (GlobalStorage || var->isConstexpr() ||
       var->mightBeUsableInConstantExpressions(Context))) {
```

https://github.com/llvm/llvm-project/pull/140542