Do not build DIE -> parent map just because we see an asm TU

Do not build DIE -> parent map just because we see an asm TU

For languages like C that don't have namespaces, we don't need to
build the DIE -> parent map, and that speeds up the analysis of the
ELF binary in the DWARF reader.

Unfortunately, just because we are seeing some MIPS assembler
translation unit, dwarf_reader::read_context::build_die_parent_maps
unnecessarily builds the DIE -> parent map, making the ELF binary
analysis slower.

This patch introduces the new function
dwarf_reader::read_context::do_we_build_die_parent_maps to make the
decision about building the DIE -> parent map in a central place.  And
it makes dwarf_reader::read_context::build_die_parent_maps use it.

With this, analysing the linux kernel becomes faster again.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>