[PATCH 2/2] gold: add a task in incremental build that can be used to check which inputs were modified

[Cary Coutant]

>>   I don't know how plug-ins interacts with this - when do plugins add
>> input objects? Currently I print an error if they do it too late.
>
> I *think* that when a plugin adds an input file we have to assume it is
> modified.  In any case this is certainly lower priority.

Plugins add their replacement files during the Plugin_hook task, which
runs after the last Read_symbols task, and before Gc_runner or
Middle_runner.

Treating a replacement file as modified sounds like the right thing.
If it was going to be unmodified, we probably wouldn't/shouldn't have
processed the file that got claimed by the plugin in the first place.
In any case, incremental linking with LTO is going to take some deep
thought.

-cary