When performing value-initialization for a class with a non-trivial,

implicitly-defined default constructor, zero-initialize the memory
before calling the default constructor. Previously, we would only
zero-initialize in the case of a trivial default constructor.

Also, simplify the hideous logic that determines when we have a
trivial default constructor and, therefore, don't need to emit any
call at all.

llvm-svn: 111779