Use that instead of rolling it up in _SwiftTypePreservingNSNumber so that we
get the right behavior when we go to write plists.
https://bugs.swift.org/browse/SR-2381
The Objective-C type encoding of Boolean values in NSNumber is that of
BOOL, which is either signed char or _Bool depending on the
platform. _SwiftTypePreservingNSNumber was using _Bool, which led to
inconsistencies when bridging vs. creating an NSNumber directly. Use
BOOL consistently.
Fixes rdar://problem/27894308.
Implements part of SE-0110. Single argument in closures will not be accepted if
there exists explicit type with a number of arguments that's not 1.
```swift
let f: (Int, Int) -> Void = { x in } // this is now an error
```
Note there's a second part of SE-0110 which could be considered additive,
which says one must add an extra pair of parens to specify a single arugment
type that is a tuple:
```swift
let g ((Int, Int)) -> Void = { y in } // y should have type (Int, Int)
```
This patch does not implement that part.
