[clang] Fix missing initializer for inline static template member with auto caused by delayed template instantiation. (PR #138122)

[Yanzuo Liu via cfe-commits]

================
@@ -6027,8 +6027,15 @@ void Sema::BuildVariableInstantiation(
   Context.setManglingNumber(NewVar, Context.getManglingNumber(OldVar));
   Context.setStaticLocalNumber(NewVar, Context.getStaticLocalNumber(OldVar));
 
+  bool VarTemplateWithAutoType = false;
+  QualType VarSourceType = OldVar->getTypeSourceInfo()->getType();
+  if (VarSourceType->getAs<AutoType>()) {
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zwuis wrote:

Can we use `->isUndeducedType()` to handle this case
```cpp
template <typename T> struct S { S(T); };
template <typename T> struct B { 
   template <typename G> inline static S var = 5; 
 };
```
?

https://github.com/llvm/llvm-project/pull/138122