Do not convert doc comments into #[doc = "..."] form unless necessary for macros

[petrochenkov]

Keep them as tokens in AST, this may partially mitigate the perf impact from turning normal comments into doc comments (#60930).

pub struct Attribute {
    pub id: AttrId,
    pub style: AttrStyle,
    pub path: Path,
    pub tokens: TokenStream,
    pub is_sugared_doc: bool,
    pub span: Span,
}

=>

pub struct Attribute {
    pub kind: AttrKind,
    pub span: Span,
    pub id: AttrId,
}

pub enum AttrKind {
    Normal(Path, TokenStream, AttrStyle),
    Doc(Symbol),
}

[nnethercote]

Because TokenStream contains a Lrc<Vec<TreeAndJoint>>, this would avoid two allocations per doc comment line: one for the Lrc and one for the Vec. That would get a big chunk of the potential saving. (We would still need one symbol interning per line, which involves a TLS access and a hash table.)

[nnethercote]

I have a local patch that changes TokenStream to a Lrc<SmallVec<[TreeAndJoint; 2]>>. It's a clear win, but it only gets rid of one of the allocations, so the approach suggested in this PR has a greater potential.

[nnethercote]

I have implemented this idea in #65750. It works well!