[Clang][Sema] Fix crash when MS dependent base class lookup occurs in an...

[Clang][Sema] Fix crash when MS dependent base class lookup occurs in an incomplete context (#83024)

When compiling the following with `-fms-compatibility`:
```
template<typename T> struct C;

// Test lookup with incomplete lookup context
template<typename T>
auto C<T>::f() -> decltype(x) { }
```
An assert fails because `CXXRecordDecl::hasAnyDependentBases` is called
on an incomplete class. This patch ensures we don't perform unqualified
lookup into dependent base classes when the lookup context is
incomplete.