swift234

- Introduce the ability for attributes to be inverted with !.
- Switch @mutable to be a tri-state attribute that is invertable with @!mutable.
- Move the semantic form of 'mutable' to being a bit on FuncDecl instead of
  something in DeclAttrs.  The former is a binary bit, the later is a tristate
  which differentiates between "not present", "present and set" "present and inverted".
- Diagnose some invalid uses of @mutable, e.g. on class methods.
- Make setters default to mutable, and allow them to be switched with @!mutable.



Swift SVN r11439