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

[via cfe-commits]

yronglin wrote:

Do we also need to update `peekNextToken` in Lexer/TokenLexer? 

> so the return value std::nullopt and tok::eof have two different meanings.

We depent std::nullopt to walk up in macro stack in `isNextTokenOneOf`.

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