make Ast_mapper iterate over all locations inside Longident.t

These locations, which are new in 5.4, are sometimes traversed (e.g. Pexp_setfield) and
sometimes not (e.g. Pexp_field). So clearly a bug.

The fix is concretely:

- change map_loc to map over both fields `{ loc; txt }` of `Location.loc`
- propagate the change, ie change `map_loc` to `map_string_loc`,
  `map_string_opt_loc` or `map_loc_lid` depending on the type. All occurrences
  of the latter are bug fixes.

Author: v-gb

CHANGES.md

@@ -4,6 +4,14 @@ Items marked with an asterisk (\*) are changes that are likely to format
 existing code differently from the previous release when using the default
 profile. This started with version 0.26.0.
 
+## unreleased
+
+### Fixed
+
+- `Ast_mapper` now iterates on *all* locations inside of Longident.t,
+  instead of only some.
+  (#2737, @v-gb)
+
 ## 0.28.1
 
 ### Highlight