gdb: allow duplicate enumerators in flag enums

gdb: allow duplicate enumerators in flag enums

I have come across some uses cases where it would be desirable to treat
an enum that has duplicate values as a "flag enum".  For example, this
one here [1]:

    enum membarrier_cmd {
            MEMBARRIER_CMD_QUERY                                = 0,
            MEMBARRIER_CMD_GLOBAL                               = (1 << 0),
            MEMBARRIER_CMD_GLOBAL_EXPEDITED                     = (1 << 1),
            MEMBARRIER_CMD_REGISTER_GLOBAL_EXPEDITED            = (1 << 2),
            MEMBARRIER_CMD_PRIVATE_EXPEDITED                    = (1 << 3),
            MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED           = (1 << 4),
            MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE          = (1 << 5),
            MEMBARRIER_CMD_REGISTER_PRIVATE_EXPEDITED_SYNC_CORE = (1 << 6),

            /* Alias for header backward compatibility. */
            MEMBARRIER_CMD_SHARED = MEMBARRIER_CMD_GLOBAL,
    };

The last enumerator is kept for backwards compatibility.  Without this
patch, this enumeration wouldn't be considered a flag enum, because two
enumerators collide.   With this patch, it would be considered a flag
enum, and the value 3 would be printed as:

  MEMBARRIER_CMD_GLOBAL | MEMBARRIER_CMD_GLOBAL_EXPEDITED

Although if people prefer, we could display both MEMBARRIER_CMD_GLOBAL
and MEMBARRIER_CMD_SHARED in the result.  It wouldn't be wrong, and
could perhaps be useful in case a bit may have multiple meanings
(depending on some other bit value).

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/include/uapi/linux/membarrier.h?id=0bf999f9c5e74c7ecf9dafb527146601e5c848b9#n125

gdb/ChangeLog:

	* dwarf2/read.c (update_enumeration_type_from_children): Allow
	flag enums to contain duplicate enumerators.
	* valprint.c (generic_val_print_enum_1): Update comment.

gdb/testsuite/ChangeLog:

	* gdb.base/printcmds.c (enum flag_enum): Add FE_TWO_LEGACY
	enumerator.