[dwarf-reader] Re-use function types inside a given TU

[dwarf-reader] Re-use function types inside a given TU

Whenever we see a function type inside a translation unit, if it
matches one that has already been seen -- i.e, one that has the same
textual representation -- we should be able to re-use that same
function type without having to compare their types to be sure they
are the same, as part of the type canonicalization process.

This slittly increases analysis speed (by a few tens of seconds on a
total of 8 minutes) by decreasing the load on type canonicalization
when anlyzing vmlinux.  It also slightly reduces memory consumption,
so I am getting it in for now.

	* src/abg-dwarf-reader.cc (istring_fn_type_map_type): Declare new
	typedef.
	(die_is_function_type): Define new static function.
	(read_context::per_tu_repr_to_fn_type_maps_): Define new data
	member ...
	(read_context::per_tu_repr_to_fn_type_maps): ... and its accessor.
	(read_context::{associate_die_repr_to_fn_type_per_tu,
	lookup_fn_type_from_die_repr_per_tu}): Define new member
	functions.
	(build_function_type): Use the new
	read_context::lookup_fn_type_from_die_repr_per_tu and
	read_context::associate_die_repr_to_fn_type_per_tu functions,
	instead of read_context::lookup_type_from_die.
	* tests/data/test-annotate/test13-pr18894.so.abi: Adjust.
	* tests/data/test-annotate/test14-pr18893.so.abi: Adjust.
	* tests/data/test-annotate/test21-pr19092.so.abi: Adjust.
	* tests/data/test-read-dwarf/test13-pr18894.so.abi: Adjust.
	* tests/data/test-read-dwarf/test14-pr18893.so.abi: Adjust.
	* tests/data/test-read-dwarf/test16-pr18904.so.abi: Adjust.
	* tests/data/test-read-dwarf/test21-pr19092.so.abi: Adjust.

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