I figured out a technique, based on a blog post by Erica Sadun (http://ericasadun.com/2014/07/28/swift-docs-generation/).
The Swift REPL has a :print_module
command that dumps all the declarations in the module. Unlike choosing a module in Xcode, it doesn't stop at just one (virtual) header.
So this command will print out all the declarations in CoreGraphics:
echo ":print_module CoreGraphics" | xcrun swift -deprecated-integrated-repl