[swift2objc] Parsing is flaky

[liamappelbe]

I'm running #1367 (with #1668 patched) on the AVFAudio module to try to see if I can migrate the ffigen objc example to swift. That'll be a good milestone to demonstrate binding a simple, but realistic, Apple API.

The most important class for this example is AVAudioPlayer, but currently it's flaky whether that class even exists in the list of declarations returned by parseAst.

[liamappelbe]

Fixed in #1668.

The main problem was that we're indicating errors by throwing exceptions. So if, for example, a method returned an enum, which we don't support yet, parseDeclaration would throw. Then since the method parser wasn't catching that exception, it also propagated up through _parseCompoundDeclaration, effectively removing that declaration. The fix is to catch the error in _parseCompoundDeclaration and just omit that method. I added tryParseDeclaration that wraps parseDeclaration, logs the exception and returns null.

More generally, any time we're calling parseDeclaration, if we want to gracefully handle a problem with the declaration, we should instead call tryParseDeclaration and handle the nullable result. I had a quick look at all the call sites and I don't think there are any other cases like this at the moment.