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

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yronglin wrote:

@Bigcheese I have a question about `A module directive may only appear as the first preprocessing tokens in a file (excluding the global module fragment.)`. IIUC, as you said in https://github.com/llvm/llvm-project/pull/90574#discussion_r1591569547, this rule intended to prohibit the following code? 
```C++
// error: User need to have a `module;` decl before any preprocessor directives.
#include "foo.h"
export module M;
```

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