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_expand/src/proc_macro.rs

@@ -162,7 +162,7 @@ impl MultiItemModifier for DeriveProcMacro {
 
         let error_count_before = ecx.dcx().err_count();
         let mut parser =
-            rustc_parse::stream_to_parser(&ecx.sess.parse_sess, stream, Some("proc-macro derive"));
+            rustc_parse::stream_to_parser(&ecx.sess.psess, stream, Some("proc-macro derive"));
         let mut items = vec![];
 
         loop {