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.
