[Explicit Module Builds] Add a setting to specify a reproducer location.

[vsapsai]

The idea is the same as Clang flag -fcrash-diagnostics-dir but for explicit module builds.

Making it a build setting instead of an environment variable on purpose. The plan is to have this setting more visible and not hidden.

rdar://164202916

[vsapsai]

I'm not particularly happy that I have reproducerOutputPath both in ClangExplicitModulesPayload and in PrecompileClangModuleTaskKey but I haven't found a better approach. Any improvements in this area are welcome.

Another questionable moment is storing reproducerOutputPath in a payload at all. I have considered making it available through some delegate (most likely through TaskExecutionDelegate). But the reproducer location is relevant only for explicitly built modules, so I've decided to put this information into a more specific class which turned out to be a payload.

[owenv]

This approach looks ok to me

[vsapsai]

@swift-ci test