Chuanqi Xu dfa7ff97b2 [C++20] [Modules] [Reduced BMI] Combine the signature of used modules ...

into the current module

Following of https://github.com/llvm/llvm-project/pull/86912. After
https://github.com/llvm/llvm-project/pull/86912, with reduced BMI, the
BMI can keep unchange if the dependent modules only changes the
implementation (without introduing new decls). However, this is not
strictly correct.

For example:

```
// a.cppm
export module a;
export inline int a() { ... }

// b.cppm
export module b;
import a;
export inline int b() { return a(); }
```

Since both `a()` and `b()` are inline, we need to make sure the BMI of
`b.pcm` will change after the implementation of `a()` changes.

We can't get that naturally since we won't record the body of `a()`
during the writing process. We can't reuse ODRHash here since ODRHash
won't calculate the called function recursively. So ODRHash will be
problematic if `a()` calls other inline functions.

Probably we can solve this by a new hash mechanism. But the safety and
efficiency may a problem too. Here we just combine the hash value of the
used modules conservatively.

2024-05-07 11:41:08 +08:00

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