Issue Description:

I would have expected the expected_rbrace_in_getset diagnostic to fire on missing a closing brace, but I don't see any test cases for it, and I can't figure out how to exercise that code path. Looks like the parsing code in parseGetSet doesn't recover gracefully in the presence of unbalanced braces. For example, the compiler trips up at the following two examples but it doesn't trigger expected_rbrace_in_getset :

struct S {
  var x : Int {
    get { 0
  }
}
// expected '}' in struct

and it also trips up on seeing other things:

struct T {
  var x : Int {
    get { 0
  }
  func f() {}
//^ expected 'get', 'set', 'willSet', or 'didSet' keyword to start an accessor definition
}

I understand that this is difficult to get right in the general case, but it would be nice to have a test case for the diagnostic, as well as have it fire more often.