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

[via cfe-commits]

yronglin 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:
> 
> ```c++
> 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.
> ```

Fixed.



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