Fix some bad behavior around static methods (#1289)

These are steps towards a fix for the problem of not being able to call a static method as follows:

    SomeType::someMethod();

One problem in the above is that the parser gets confused and parses an (anonynmous!) function declaration. This change doesn't address that problem, but *does* fix the problem that when checking fails to coerce `SomeType::someMethod` into a type it was triggering an unimplemented-feature exception rather than a real error message.

Another problem was that if the above is re-written to try to avoid the parser bug:

    (SomeType::someMethod)();

we end up with a call where the base expression (the callee) is a `ParenExpr` and the code for handling calls wasn't expecting that. Instead, it sent the overload resolution logic into an unimplemented case that was bailing by throwing an arbitrary C++ exception instead of emitting a diagnostic.

This latter issue was fixed in two ways. First, the code path that failed to emit a diagnostic now emits a reasonable one for the unimplemented feature (this still ends up being a fatal compiler error). Second, we properly handle the case of trying to call a `ParenExpr` by unwrapping it and using the base expression instead, so that `(<func>)(<args>)` is always treated the same as `<func>(<args>)`.

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