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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

It seems that the code is currently checking for the `module` or `import` at the start of the line _after_ macro expansion:
```cpp
#define IMPORT import
template <typename T>
struct import;

extern
IMPORT<int> a;
```
is supposed to compile (based on the wording).

Accepted with `IMPORT`: https://godbolt.org/z/v16sEd5jK
Rejected with `import` (except for Clang, which is waiting for this PR): https://godbolt.org/z/Gz11axThf

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