[gold][patch] Reduce heap usage for string merge sections

[Cary Coutant <ccoutant at google dot com>]

I used tcmalloc to profile the heap usage of gold while building a
large binary compiled with -O2 and -g, and found that the
Merged_strings structures were responsible for just over 50% of the
total heap used by gold. That structure has 6 fields: a pointer to the
object, the input section index, the input offset, a pointer to the
string itself, the length of the string, and the stringpool key. The
string field was never used, so I removed it. The pointer to the
object and the input section index were common across fairly large
lists of strings, so I separated the one list per output section into
separate lists for each (object, shndx), and removed those two fields
from the Merged_string struct. The length field was easily recomputed
by subtracting adjacent input offsets, so I removed it, too. The
result is a 67% reduction in the size of the Merged_string structure,
and the net effect was to reduce the total heap used for my benchmark
from 3.45 GiB to 2.56 GiB (25% less heap).

The Object_merge_map structure is now responsible for more heap than
anything else; I'll be taking a look at reducing its memory footprint
next.

Tested on x86_64 Linux. OK?

-cary

        * merge.h (Output_merge_string::Merged_string): Remove object, shndx,
        string, and length fields.
        (Output_merge_string::Merged_strings_list): New type.
        (Output_merge_string::Merged_strings_lists): New typedef.
        (Output_merge_string): Replace merged_strings_ with
        merged_strings_lists_.
        * merge.cc (Output_merge_string::do_add_input_section): Allocate new
        Merged_strings_list per input object and section.  Don't store pointer
        to the string.  Don't store length with each merged string entry.
        (Output_merge_string::finalize_merged_data): Loop over list of merged
        strings lists.  Recompute length of each merged string.