diagnostics: fix the order of multiple declarations reports

On

  %token FOO "foo"
  %printer {} "foo"
  %printer {} FOO

we report

  /tmp/foo.yy:2.10-11: error: %printer redeclaration for FOO
   %printer {} "foo"
            ^^
  /tmp/foo.yy:3.10-11:     previous declaration
   %printer {} FOO
            ^^

* src/symtab.c (locations_sort): New.
Use it.
* tests/input.at (Invalid Aliases): Stress the order of diagnostics.