Don't inherit range-type signed-ness from underlying type

Don't inherit range-type signed-ness from underlying type

A recent commit changed gdb to inherit the signed-ness of a range type
from its underlying type:

    commit cfabbd351a174406fd5aa063303f5c8bf9266bbc
    Author: Tom Tromey <tom@tromey.com>
    Date:   Sat Oct 17 11:41:59 2020 -0600

      Make range types inherit signed-ness from base type

This passed testing -- but unfortunately, additional testing at
AdaCore showed that this change was incorrect.  GNAT, at least, can
emit an unsigned range type whose underlying type is signed.

This patch reverts the code change from the above.  I chose not to
reintroduce the FIXME comments, because now we know that they are
incorrect.  Instead, this patch also adds a comment to
create_range_type.

A new test case is included as well.

2020-10-26  Tom Tromey  <tromey@adacore.com>

	* gdbtypes.c (create_range_type): Revert previous patch.  Add
	comment.

gdb/testsuite/ChangeLog
2020-10-26  Tom Tromey  <tromey@adacore.com>

	* gdb.ada/unsigned_range/foo.adb: New file.
	* gdb.ada/unsigned_range/pack.adb: New file.
	* gdb.ada/unsigned_range/pack.ads: New file.
	* gdb.ada/unsigned_range.exp: New file.