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

[via cfe-commits]

yronglin wrote:

Hmm, the code format suggestion looks strange, PR did not change this line :-)
```diff
diff --git a/clang/include/clang/Lex/Token.h b/clang/include/clang/Lex/Token.h
index 2b30f52a0..764673bca 100644
--- a/clang/include/clang/Lex/Token.h
+++ b/clang/include/clang/Lex/Token.h
@@ -86,7 +86,7 @@ public:
                                 // macro stringizing or charizing operator.
     CommaAfterElided = 0x200, // The comma following this token was elided (MS).
     IsEditorPlaceholder = 0x400, // This identifier is a placeholder.
-    IsReinjected = 0x800, // A phase 4 token that was produced before and
+    IsReinjected = 0x800,        // A phase 4 token that was produced before and
                           // re-added, e.g. via EnterTokenStream. Annotation
                           // tokens are *not* reinjected.
     FirstPPToken = 0x1000, // This token is the first pp token in the
```

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