This patch is extracted from D24267 which implements the --gdb-index option.
(aim is to reduce and split review parts)
In this patch partial gdb_index section is created.
For costructing the .gdb_index section 6 steps should be performed (details are in
SplitDebugInfo.cpp file header), this patch do first 3:
- Creates proper section header.
- Fills list of compilation units.
- Types CU list area is not supposed to be supported, so it is ignored and therefore
can be treated as implemented either.