Whenever a variable is written without an initializer,
default-initialize the variable (e.g., by calling the default
constructor) or produce an error. Fixes <rdar://problem/13134629>,
<rdar://problem/11941389>, and the majority of
<rdar://problem/11619148>.
Missing still is the default initialization of struct members within
user-written constructors. That's the last part of
<rdar://problem/11619148>.
Swift SVN r4974
