[C++20] [Modules] Only diagnose the non-inline external variable

definitions in header units

Address part of https://github.com/llvm/llvm-project/issues/60079.

Since the the declaration of a non-inline static data member in its
class definition is not a definition. The following form:

```
class A {
public:
    static const int value = 43;
};
```

should be fine to appear in a header unit. From the perspective of
implementation, it looks like we simply forgot to check if the variable
is a definition...

Reviewed By: iains

Differential Revision: https://reviews.llvm.org/D141905