Macro `literal` fragment specifiers should forward to exact tokens about rust

[petrochenkov]

This is a consequence of - NUM being supported by literal.
literal may contain multiple tokens, so it's wrapped into a delimited group to preserve parsing priorities, similarly to expr, ty and others.
Only tt and ident always represent single tokens and therefore not wrapped.

Some PR with backround discussion - #92472.
Relevant part of the reference - https://doc.rust-lang.org/reference/procedural-macros.html#declarative-macro-tokens-and-procedural-macro-tokens.

Note that the reference deliberately says

Such token streams may be wrapped into delimited groups (Group) with implicit delimiters (Delimiter::None) when it's necessary for preserving parsing priorities.

to accommodate for potential future changes (e.g. omitting grouping for single-token streams).
Whether it can be done backward compatibly in practice is a separate question.

[tgross35]

@rustbot label +A-macros +WG-macros +A-maybe-future-edition +T-lang -needs-triage +A-macros-2.0

[tgross35]

@rustbot label +A-macros-2.0