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

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yronglin wrote:

The only difference between https://github.com/llvm/llvm-project/pull/143898 and this PR is https://github.com/llvm/llvm-project/pull/145244/commits/6fe2cd4c723695c0ce6a4d2ced7a2b2628cdaf5d.

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