[PrintAsClang] [NFC] Refactor decl sorting

Factor ModuleContentsWriter’s declaration sorting out into a separate helper function, and additionally rework that function into a series of abstract comparisons supported by various helper functions. This declutters the function and makes the high-level logic it implements much more clear, at the cost of hiding much of the control flow inside a macro.

This also makes a very small change to the handling of generic signature comparisons: declarations without a generic signature will be factored into comparisons by comparing an empty string.

Author: beccadax