The test was inspired by locally running the query against files in
https://github.com/apple/swift/tree/main/test/Parse
A query for missing elements was also added to the AST tests, expecting
nothing to be found.
As `ASTMangler` crashes when called on `ModuleDecl`, we simply use
its name.
This might probably not work reliably in a scenario where multiple
modules are compiled with the same name (like `main`), but this is left
for future work. At the moment this cannot create DB inconsistencies.
