[C++20] [Modules] Make the linkage consistent for template and its

specialization

Before the patch, the compiler would crash for the test due to
inconsistent linkage.

This patch tries to avoid it by make the linkage consistent for template
and its specialization. After the patch, the behavior of compiler would
be partially correct for the case.
The correct one is:

```
export template<class T>
void f() {}

template<>
void f<int>() {}
```

In this case, the linkage for both declaration should be external (the
wording I get by consulting in WG21 is "the linkage for name f should be
external").

And for the case:
```
template<class T>
void f() {}

export template<>
void f<int>() {}
```

Compiler should reject it. This isn't done now. After all, this patch would
stop a crash.

Reviewed By: iains, aaron.ballman, dblaikie

Differential Revision: https://reviews.llvm.org/D120397

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