swift234

fix <rdar://problem/19874152> Swift 1.2 struct memberwise initializer violates new sanctity of previously set `let` property

If a non-static 'let' field in a struct has an initializer, its implicitly generated
memberwise initializer should not override that.  Handle this by having the memberwise initializer
and default initializer use the initial value of the property instead of an argument to the init.

This would have been a lot easier if sema was synthesizing the ctor bodies instead of SILGen,
but here it is.  The missing case is narrow (let (x,y) = (1,2)) and for now we just disable
implicit synthesization of the incorrect case.



Swift SVN r25424