[clang] [C++20][Modules] Implement P1857R3 Modules Dependency Discovery (PR #107168)

[Hubert Tong via cfe-commits]

https://github.com/hubert-reinterpretcast commented:

This needs tests for `-E`.

The line separation is not getting maintained:
```cpp
export module m; int x;
extern "C++" int *y = &x;
```

Output:
```
# 1 "<stdin>"
# 1 "<built-in>" 1
# 1 "<built-in>" 3
# 665 "<built-in>" 3
# 1 "<command line>" 1
# 1 "<built-in>" 2
# 1 "<stdin>" 2
export module m; int x;extern "C++" int *y = &x;
```

https://github.com/llvm/llvm-project/pull/107168