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

[Hubert Tong via cfe-commits]

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

A user-defined string literal is not a _string-literal_:
```cpp
// expected-no-diagnostics
#define STR(X) # X
const char *s = STR(
import "hello"_world;
);
```

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