Replace "early parsed" terminology

AST attributes use "early parsed"/"parsed" terminology to refer to the
`CfgTrace` and `CfgAttrTrace` attributes. I think this terminology is
meant to echo the terminology used for HIR attributes, i.e.
`hir::Attribute::{Parsed,Unparsed}`, probably because
`hir::Attribute::Parsed` is used for attributes that aren't stored in a
token-based form.

But this naming is misleading. "Early parsed" attributes aren't parsed
at all because they are inserted by the compiler and cannot be written
in source code. There are also two comments that claim that these
attributes are kept in parsed form "so they don't have to be reparsed
every time they're used, for performance", which is simply incorrect.

This commit renames these as "synthetic" attributes, which better
reflects their nature. The commit also fixes the incorrect comments.

Note that `is_parsed_attribute` is unchanged, because it refers to the
HIR attribute meaning. (And the removal of the synthetic attributes from
it in the previous commit is now more obviously correct.)