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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

> Anyhow, even without throwing CWG2947 into the mix, stuff like this is allowed:
> 
> ```c++
> export module x _Pragma("GCC warning \"Hi\"");
> ```

The above is working, but the seemingly simpler case is not:
```cpp
export module x; _Pragma("GCC warning \"hi\"");
```

Gives:
```
<stdin>:1:18: warning: extra tokens at end of 'module' directive [-Wextra-tokens]
    1 | export module x; _Pragma("GCC warning \"hi\"");
      |                  ^
      |                  //
<stdin>:1:18: error: a type specifier is required for all declarations
1 warning and 1 error generated.
```

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