[cxx-interop] Emit type metadata for foreign types more often

[Xazax-hun]

Metadata for foreign types are emitted lazily, when SILGen generates a reference to it. Unfortunately, C++ reverse interop can also introduce references to such metadata in the generated header when types are used as generic arguments. This adds a type visitor to make note of the type metadata use for those generic arguments in public APIs when C++ interop is enabled. Fixes #75593

rdar://132925256

[Xazax-hun]

@swift-ci please smoke test

[Xazax-hun]

@swift-ci please smoke test

[Xazax-hun]

Looks like emitting type metadata for foreing types on Windows is not supported at the moment.

@swift-ci please smoke test

[ADKaster]

surface level from me, thanks for looking at my reported issue!

[Xazax-hun]

@swift-ci please smoke test

[compnerd]

Is there a reason that this is not supported on Windows?

[egorzhdan]

My understanding is that on Windows we end up requiring a metadata completion function for C++ foreign reference types. That logic just isn't implemented yet for C++ types.

swift/lib/IRGen/GenMeta.cpp

Lines 6636 to 6639 in bd313de

	void emitInitializeMetadata(IRGenFunction &IGF, llvm::Value *metadata,
	MetadataDependencyCollector *collector) {
	llvm_unreachable("Not implemented for foreign reference types.");
	}

[Xazax-hun]

@swift-ci please smoke test

[Xazax-hun]

Only rerunning tests because the last completion was a long time ago (to make sure it does not conflict with anything added since then).

[Xazax-hun]

@swift-ci please smoke test windows

[rjmccall]

This is acceptable as a short-term workaround, but in the long term we don't want to be doing this. It is an important goal of C++ interop to be able to work with an arbitrary existing Swift module, which means we must not require the module to be emitted in a special way. Maybe we need some ability to emit a supporting .o when emitting a C++ interop header.