[clang] [Modules] No transitive source location change (PR #86912)

[Chuanqi Xu via cfe-commits]

ChuanqiXu9 wrote:

 BTW, after this patch, with reduced BMI (https://github.com/llvm/llvm-project/pull/85050), we can already do something more interesting than reformating:

```
//--- A.cppm
export module A;
int funcA0();
int funcA1();
export int funcA() {
    return funcA0();
}

//--- A.v1.cppm
export module A;

int funcA0();
int funcA1();
export int funcA() {
    return funcA0() + funcA1();
}

//--- B.cppm
export module B;
import A;

export int funcB() {
    return funcA();
}
```

Now the B.pcm will keep unchanged  with `A.pcm` from `A.cppm` and `A.v1.pcm` from `A.v1.cppm`. We changed the implementation of `funcA()` from `return funcA0();` to `return funcA0() + funcA1();`. And the `B.pcm` can still get the same content.

https://github.com/llvm/llvm-project/pull/86912