[PrintAsObjC] Separate out module contents printing too

[jrose-apple]

The top-level PrintAsObjC now just covers import printing and the near-literal prologue/epilogue text, while the decl printing (and import collecting) is in its own file. No functionality change.

(Previously: #27163)

[jrose-apple]

Note: The PrintAsObjC.cpp diff is easier to read if you ignore whitespace changes.

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[jrose-apple]

@swift-ci Please test source compatibility

[varungandhi-apple]

I'm slightly surprised at the line count difference -- eyeballing the changes would've made me think it overall adds <50 lines, but there is a net +82 lines.

[jrose-apple]

I'm slightly surprised at the line count difference -- eyeballing the changes would've made me think it overall adds <50 lines, but there is a net +82 lines.

Some lines were changed to protect the innocent adjust to the new interface. I think that accounts for it.

[jrose-apple]

@swift-ci Please smoke test macOS