[clang] Reapply "[C++20][Modules] Implement P1857R3 Modules Dependency Discovery" (#173130)" (PR #173789)

[Corentin Jabot via cfe-commits]

================
@@ -776,6 +831,12 @@ class Preprocessor {
   /// Only one of CurLexer, or CurTokenLexer will be non-null.
   std::unique_ptr<Lexer> CurLexer;
 
+  /// Lexers that are pending destruction, deferred until the current
+  /// Lex call stack unwinds completely (LexLevel returns to 0).
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cor3ntin wrote:

```suggestion
  /// stack of Lexer unwinds completely (LexLevel returns to 0).
```

https://github.com/llvm/llvm-project/pull/173789