Use field init shorthand where possible

Field init shorthand allows writing initializers like `tcx: tcx` as
`tcx`. The compiler already uses it extensively. Fix the last few places
where it isn't yet used.

rustfmt.toml

@@ -4,6 +4,7 @@ use_small_heuristics = "Max"
 merge_derives = false
 group_imports = "StdExternalCrate"
 imports_granularity = "Module"
+use_field_init_shorthand = true
 
 # Files to ignore. Each entry uses gitignore syntax, but `!` prefixes aren't allowed.
 ignore = [