Add noreturn annotations

A method of the form:

```
- (void)assertsPreventNullFunctionPointerAnalyzerWarnings:(void (^)())block {
    FCYAssertParameterNotNil(block);
    block();
}
```

generates analyzer warnings that `block` may be a null function pointer despite the `FCYAssertParameterNotNil` check because the analyzer cannot detect that the assert will prevent execution from proceeding in that case.

This branch adds an annotation to instruct the analyzer to assume assert calls do not return and therefore correctly determine that this code path in unreachable if `block` is null.

Also updated the `.gitignore` to exclude user-specific Xcode files which changed while I was creating this change.