Eliminate confusing "globals" terminology.

There are some structures that are called "globals", but are they global
to a compilation session, and not truly global. I have always found this
highly confusing, so this commit renames them as "session globals" and
adds a comment explaining things.

Also, the commit fixes an unnecessary nesting of `set()` calls
`src/librustc_errors/json/tests.rs`

src/test/ui-fulldeps/mod_dir_path_canonicalized.rs

@@ -19,7 +19,7 @@ use std::path::Path;
 mod gravy;
 
 pub fn main() {
-    rustc_ast::with_default_globals(|| parse());
+    rustc_ast::with_default_session_globals(|| parse());
 
     assert_eq!(gravy::foo(), 10);
 }

src/test/ui-fulldeps/pprust-expr-roundtrip.rs

@@ -208,7 +208,7 @@ impl MutVisitor for AddParens {
 }
 
 fn main() {
-    rustc_ast::with_default_globals(|| run());
+    rustc_ast::with_default_session_globals(|| run());
 }
 
 fn run() {