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_codegen_ssa/src/assert_module_sources.rs

@@ -176,7 +176,7 @@ impl<'tcx> AssertModuleSource<'tcx> {
     /// Scan for a `cfg="foo"` attribute and check whether we have a
     /// cfg flag called `foo`.
     fn check_config(&self, attr: &ast::Attribute) -> bool {
-        let config = &self.tcx.sess.parse_sess.config;
+        let config = &self.tcx.sess.psess.config;
         let value = self.field(attr, sym::cfg);
         debug!("check_config(config={:?}, value={:?})", config, value);
         if config.iter().any(|&(name, _)| name == value) {