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.
