[clang] [clang-tools-extra] [clangd] Add an explicit `IsCachingLexMode` flag to fix preprocessor caching-lexer state tracking (PR #203716)

[Corentin Jabot via cfe-commits]

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@@ -1190,6 +1190,9 @@ class Preprocessor {
   /// Lex() should be invoked.
   CachedTokensTy::size_type CachedLexPos = 0;
 
+  /// True when the caching lexer is installed as the active lexer layer.
+  bool IsCachingLexMode = false;
+
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cor3ntin wrote:

Can we just test `CurLexerCallback == CLK_CachingLexer` instead of adding new state?

https://github.com/llvm/llvm-project/pull/203716