Attributes merged without space

[bnbarham]

The following code:

#if os(macOS)
@available(macOS, unavailable)
@_spi(Foo)
#endif
public let myVar = "Test"

Is rewritten as:

#if os(macOS)
  @available(macOS, unavailable)@_spi(Foo)
#endif
public let myVar = "Test"

The #if here is the important part, we don't reflow at all otherwise. Ideally we shouldn't join at all, but if we do, we should at least add a space.

(mirrors rdar://133869717)