Instead of checking that the stdlib can be loaded in a variety of places, check it when setting up the compiler instance. This required a couple more checks to avoid loading the stdlib in cases where it’s not needed.
To be able to differentiate stdlib loading failures from other setup errors, make `CompilerInstance::setup` return an error message on failure via an inout parameter. Consume that error on the call side, replacing a previous, more generic error message, adding error handling where appropriate or ignoring the error message, depending on the context.
We had some situations left that neither returned an error, nor called the callback with results in `performOperation`. Return an error in these and adjust the tests to correctly match the error.
For example, if stdlib is not found, CompilerInstance simply returns
before creating PersistentParserState. In such cases, completion should
just return empty result.
Previously, it caused SourceKit crash.
rdar://problem/58663066
