[clang] [clang][Preprocessor] Handle the first pp-token in EnterMainSourceFile (PR #145244)

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yronglin wrote:

> Compile-time: https://llvm-compile-time-tracker.com/compare.php?from=40eee8ec7fb64dd1c6f15e8cd6e087f5d9f9d37e&to=2c67921274cfa31ccf7c59789df153fabc601816&stat=instructions:u

Thanks! This seems to relieve the performance regression.

https://github.com/llvm/llvm-project/pull/145244