Added support to BOLT for DW_OP_GNU_push_tls_address. So now
DW_TAG_variable with this OP in DW_AT_location will appear in debug
names acceleration table. Although not in the DWARF 5 spec it is similar
to DW_OP_form_tls_address. Without this support llvm-dwarfdump --verify
--debug-names will report errors.

This fix handles a case where a DIE that does not have
DW_AT_name/DW_AT_linkage_name, but has a reference to another DIE using
DW_AT_abstract_origin/DW_AT_specification. It also fixes a bug where
there are cross CU references for those attributes. Previously it would
use a DWARF Unit of a DIE which was being processed The
warf5-debug-names-cross-cu.s test just happened to work because how it
was constructed where string section was shared by both DWARF Units.

To resolve DW_AT_name/DW_AT_linkage_name this patch iterates over
references until it either reaches the final DIE or finds both of those
names.

Adding support for DW_TAG_union_type for DebugNames acceleration tables.

There could be multiple TUs with the same hash in various DWO files. In
bigger binaries this could be in the thousands. Although they could be
structurally different and we need to output Entries for all of them,
for the purposes of figuring out a TU hash we only need one entry in
Foreign TU list.

Previously when an entry was skipped in parent chain a child will point
to the next valid entry in the chain. After discussion in
https://github.com/llvm/llvm-project/pull/91808 this is not very useful.
Changed implemenation so that all the children of the entry that is
skipped won't have DW_IDX_parent.

Previously BOLT was only doing it for DW_TAG_variables. It looks like
other type of DIEs can have this. So making it global.

The DW_AT_abstract_origin can be a cross-cu reference as a by-product of
LTO. On IR level for absolute references an address is stored, vs a DIE
for relative references. Added a map to keep track of cross-cu
referenced DIEs to use when we add an Entry.

According to the DWARF spec a DIE that has DW_AT_specification or
DW_AT_abstract_origin can be part of .debug_name if a DIE those
attribute points to has DW_AT_name or DW_AT_linkage_name.

This adds support for DW_IDX_parent. If DIE has a parent then
DW_IDX_parent in Entry will point to Entry for that parent DIE.
Otherwise it will have DW_FORM_flag_present in abbrev. Which takes zero
space in Entry.

This came from

https://discourse.llvm.org/t/rfc-improve-dwarf-5-debug-names-type-lookup-parsing-speed/74151

The foreign TU list immediately follows the local TU list and they both
use the same index, so that if there are N local TU entries, the index
for the first foreign TU is N.

Changed so that the size of local TU is accounted for when setting
foreign TU index.