[cxx-interop] Fix crash when importing C++ forward-declared template specializations in typedefs (#84186)
Swift crashes when importing C++ typedefs to forward-declared explicit
template specializations. In assert build we see this assertion failure
happening in clang:
`Assertion failed: (!T->isDependentType() && "should not see dependent
types here"), function getTypeInfoImpl, file TypeNodes.inc, line 79.`
Example that crashes:
```cpp
template <typename T> struct MyTemplate { T value; };
template <> struct MyTemplate<int>; // Forward-declared specialization
typedef MyTemplate<int> MyIntTemplate;
```
In this patch, I propose detecting forward-declared explicit
specializations in typedef imports. Instead of crashing, these
specializations should be blocked from being imported with a diagnostic
similar to the forward declared (but not defined) structs.
rdar://147595723
swift
cc8f0605 - [cxx-interop] Fix crash when importing C++ forward-declared template specializations in typedefs (#84186)
