Rename all ParseSess variables/fields/lifetimes as psess.

Existing names for values of this type are `sess`, `parse_sess`,
`parse_session`, and `ps`. `sess` is particularly annoying because
that's also used for `Session` values, which are often co-located, and
it can be difficult to know which type a value named `sess` refers to.
(That annoyance is the main motivation for this change.) `psess` is nice
and short, which is good for a name used this much.

The commit also renames some `parse_sess_created` values as
`psess_created`.

compiler/rustc_parse/src/lexer/unicode_chars.rs

@@ -350,7 +350,7 @@ pub(super) fn check_for_substitution(
 
     let Some((_, ascii_name, token)) = ASCII_ARRAY.iter().find(|&&(s, _, _)| s == ascii_str) else {
         let msg = format!("substitution character not found for '{ch}'");
-        reader.sess.dcx.span_bug(span, msg);
+        reader.psess.dcx.span_bug(span, msg);
     };
 
     // special help suggestion for "directed" double quotes