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.
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.
DWARF5 spec supports the .debug_names acceleration table. This is the
formalized version of combination of gdb-index/pubnames/types. Added
implementation of it to BOLT. It supports both monolothic and split
dwarf, with and without Type Units. It does not include parent indices.
This will be in followup PR. Unlike LLVM output this will put all the
CUs and TUs into one Module.
