Query the SourceLookupCache for the operator decls,
and use ModuleDecl::getOperatorDecls for both
frontend stats and to clean up some code
completion logic.
In addition, this commit switches getPrecedenceGroups
over to querying SourceLookupCache.
A bunch of tests that technically require Objective-C interop were
not labeled as such, and worked in the absence of Objective-C interop
due to bugs in the type checker.
