Keep original literal tokens in AST
The original literal tokens (`token::Lit`) are kept in AST until lowering to HIR.
The tokens are kept together with their lowered "semantic" representation (`ast::LitKind`), so the size of `ast::Lit` is increased (this also increases the size of meta-item structs used for processing built-in attributes).
However, the size of `ast::Expr` stays the same.
The intent is to remove the "semantic" representation from AST eventually and keep literals as tokens until lowering to HIR (at least), and I'm going to work on that, but it would be good to land this sooner to unblock progress on the [lexer refactoring](https://github.com/rust-lang/rust/pull/59706).
Fixes a part of https://github.com/rust-lang/rust/issues/43081 (literal tokens that are passed to proc macros are always precise, including hexadecimal numbers, strings with their original escaping, etc)
Fixes a part of https://github.com/rust-lang/rust/issues/60495 (everything except for proc macro API doesn't need escaping anymore)
This also allows to eliminate a certain hack from the lexer (https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/pretty-printing.20comments/near/165005357).
cc @matklad
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