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

[Hubert Tong via cfe-commits]

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

There is a separate regression in this PR. Consider:
```cpp
import <=>;
```

With this PR, we get:
```
$ clang++ importHeaderToken.cc -std=c++26 -fsyntax-only
importHeaderToken.cc:1:1: error: the import directive is ill-formed, import keyword must be immediately followed on the same line by an identifier, '<', '"', or ':',
      but not '::', after being at the start of a line or preceded by an export at the start of the line
    1 | import <=>;
      | ^
importHeaderToken.cc:1:8: error: expected module name
    1 | import <=>;
      |        ^
2 errors generated.
Return:  0x01:1   Wed Sep 17 23:48:22 2025 EDT
```

Without it, we get:
```
importHeaderToken.cc:1:8: fatal error: '=' file not found
    1 | import <=>;
      |        ^~~
1 error generated.
Return:  0x01:1   Wed Sep 17 23:48:26 2025 EDT
```

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