swift234

fix two QoI issues:
 - <rdar://problem/16306600> QoI: passing a 'let' value as an inout results in an unfriendly diagnostic
 - <rdar://problem/16927246> provide a fixit to change "let" to "var" if needing to mutate a variable

We now refer to an inout argument as such, e.g.:

t.swift:7:9: error: cannot pass 'let' value 'a' as inout argument
  swap(&a, &b)
        ^

we also produce a note with a fixit to rewrite let->var in trivial cases where mutation is
being assed for, e.g.:

t.swift:3:3: note: change 'let' to 'var' to make it mutable
  let a = 42
  ^~~
  var

The note is produced by both Sema and DI.



Swift SVN r27774