This mode is similar to `swift-symbolgraph-extract`; it takes a subset of compiler
flags to configure the invocation for module loading, as well as a module name
whose contents should be extracted. It does not take any other input files. The
output is a single text file specified by `-o` (or `stdout` if not specified).
While the most common use case for this would be viewing the synthesized Swift
interface for a Clang module, since the implementation simply calls
`swift::ide::printModuleInterface` under the hood, it's usable for any module
that Swift can import. Thus, it could also be used to view a synthesized textual
representation of, say, a compiled `.swiftmodule`.
One could imagine that in the future, we might add more flags to
`swift-synthesize-interface` to modify various `PrintOptions` used when
generating the output, if we think those would be useful.
