[C++20] [Modules] Add signature to the BMI recording export imported

modules

After https://github.com/llvm/llvm-project/pull/86912,
for the following example,

```
export module A;
export import B;
```

The generated BMI of `A` won't change if the source location in `A`
changes. Further, we plan avoid more such changes.

However, it is slightly problematic since `export import` should
propagate all the changes.

So this patch adds a signature to the BMI of C++20 modules so that we
can propagate the changes correctly.