In a mixed Objective-C / Swift module, we have a Clang module overlay that’s a Source file, not a serialized AST as is currently assumed. That assumption caused a crash when retrieving the symbol graph as part of a cursor info request to SourceKit, which was invoked on a method defined in the Objective-C part of the module.
To fix the crash, recursively use the same logic that already exists to serialize a module to also serialize the clang overlay module since that function alreayd correctly handles the distinction between source files and serialized ASTs.
Resolves rdar://76951147
Unless you're familiar with the way the swift source code is organized,
it's not clear what "SourceDocInfo" means, or how it is different from
DocSupport, etc. Move the tests into directories that are named based
on their request (note: we already had one test under CursorInfo, which
just made things even more confusing).
