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

[Hongyu Chen via llvm-commits]

yugier 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`.

Sounds good! I will have a new branch and PR for remove `maybeSplitExpr` and then come back to `struct Token` based on the new stateful Lexer.

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