Fix FDE indexing while scan debug_info section.

There are some differences between eh_frame and debug_frame formats that
are not considered by DWARFCallFrameInfo::GetFDEIndex. An FDE entry
contains CIE_pointer in debug_frame in same place as cie_id in eh_frame.
As described in dwarf standard (section 6.4.1), CIE_pointer is an
"offset into the .debug_frame section". So, variable cie_offset should
be equal cie_id for debug_frame.

FDE entries with zeroth CIE pointer (which is actually placed in cie_id
variable) shouldn't be ignored also.

I have also added a little change which allow to use debug_info section
when eh_frame is absent. This case really can take place on some platforms.

Patch from tatyana-krasnukha.
https://reviews.llvm.org/D33504

llvm-svn: 303847