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

[via cfe-commits]

================
@@ -912,6 +931,10 @@ bool Scanner::lexPPLine(const char *&First, const char *const End) {
     CurDirToks.clear();
   });
 
+  // FIXME: Shoule we handle @import as a preprocessing directive?
----------------
yronglin wrote:

@Bigcheese I have disabled handle `@import` as a pp-directive in dependency scanner, this may cause the following thing:

https://github.com/llvm/llvm-project/blob/c945022f2fd8321559d84e2272005487c5ced924/clang/lib/Parse/Parser.cpp#L2526

Currently we try to eat a eod token in parser, This doesn't seem to make sense, the eod token should consumed in preprocessor(This eod token occurred becaused we handle @import as a pp-directive in dependency scanner, but not do the same thing in preprocessor.).

IIUC, I think we have 2 options:

1. Don't handle `@import` as a pp-directive.
2. In both dependency scanner and preprocessor LexAfterModuleImport, handle  `@import` as a pp-directive.

What do you think?

Do you know where can we find the standard of objective-c? :) 

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