[nfc] [lldb] Align `user_id_t` format to the current `DIERef` format

Current user_id_t format is:
        63{isDebugTypes} 62..32{dwo || 7fffffff}
        31..0 {die_offset}
while current DIERef format is (I have made up the bit positions but the
field widths do match):
        63{m_section==isDebugTypes} 62{m_dwo_num_valid} 61..32{m_dwo_num}
        31..0 {m_die_offset}

Proposing to change user_id_t to:
        63{isDebugTypes} 62{dwo_is_valid} 61..32{dwo; 0 if !valid}
        31..0 {die_offset}

There is no benefit of having 31-bits wide dwo_num in user_id_t when it
gets converted to 30-bits width in DIERef.

This patch is for future DWZ patchset which extends the dwo_is_valid bit
into a 2-bit field (normal, DWO, DWZ, DWZcommon) so that both user_id_t
and DIERef can be changed then the same way.

It would be best to somehow unify user_id_t and DIERef but I do not plan
to do that. user_id_t should probably remain a number for the Python API
compatibility while there still needs to be some class with all the
methods to access it.

SymbolFileDWARF::GetDwpSymbolFile() and SymbolFileDWARF::GetDIE use
0x3fffffff for DWP but that does not clash:

formerly:
  31bits32..62:0x7fffffff = normal unit / not any DWO
  31bits32..62:0x3fffffff = DWP
  31bits32..62:others = DWO unit number

after this patch:
  bit62=0 30bits32..61:any = normal unit / not any DWO
  bit62=1 30bits32..61:0x3fffffff = DWP
  bit62=1 30bits32..61:others = DWO unit number

Differential Revision: https://reviews.llvm.org/D90413