[clang] [clang][Preprocessor] Add peekNextPPToken, makes look ahead next token without side-effects (PR #143898)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

I'd like @AaronBallman and @Bigcheese to look at this but it looks reasonable

https://github.com/llvm/llvm-project/pull/143898