[lld] [ELF] Added `struct Token` and changed `next()` and `peek()` to return Token (PR #100180)

[Fangrui Song via llvm-commits]

MaskRay wrote:

The lexer representation should probably be made stateful (and remove `inExpr`) before accepting a StringRef => Token change like this, otherwise I am concerned with mistaking certain identifiers as keywords (e.g. `INFO`).

I'll take a stab at removing `maybeSplitExpr`.

https://github.com/llvm/llvm-project/pull/100180