[gdb/ada] Extend type equivalence test in ada_resolve_enum

[gdb/ada] Extend type equivalence test in ada_resolve_enum

When running test-case gdb.ada/local-enum.exp with target board debug-types, I
run into:
...
(gdb) print v1(three)^M
No name 'three' in enumeration type 'local__e1'^M
(gdb) FAIL: gdb.ada/local-enum.exp: print v1 element
...

The array V1 is of type A1 which is an array with index type E1, containing
"three" as enumerator:
...
  type E1 is (one, two, three);
  type A1 is array (E1) of Integer;
  V1 : A1 := (0, 1, 2);
...

There's also a type E2 that contains three as enumerator:
...
  type E2 is (three, four, five);
...

When doing "print v1(three)", it's the job of ada_resolve_enum to resolve
"three" to type E1 rather than type E2.

When using target board debug-types, the enums E1 and E2 are replicated in the
.debug_types section, and consequently in ada_resolve_enum the type
equivalence check using a pointer comparison fails:
...
  for (int i = 0; i < syms.size (); ++i)
    {
      /* We already know the name matches, so we're just looking for
	 an element of the correct enum type.  */
      if (ada_check_typedef (syms[i].symbol->type ()) == context_type)
 	return i;
    }
...

Fix this by also trying a structural comparison using
ada_identical_enum_types_p.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR ada/29335
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29335