[patch] Use a std::vector instead of a std::map to hold Input_merge_map

[Rafael Espíndola]

A std::map is hardly the best data structure for a small map from
small integers.

The attached patch uses a std::vector<std::pair>> instead.

This simplifies the code and speeds up linking of chromium (see
attached perf logs).

Cheers,
Rafael

2015-04-23  Rafael Ávila de Espíndola <rafael.espindola@gmail.com>

* merge.cc (get_input_merge_map): Update for data structure change.
(get_or_make_input_merge_map): Update for data structure change.
* merge.h (Object_merge_map): Use a std::vector<std::pair>> instead of
a std::map.
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