Relax the construction of a definition for implicit, trivial default

constructors. We perform semantic checking when creating the definition, and
this isn't needed in certain contexts (value initialization) but is in others
(default initialization). This fixes PR7948.

We add explicit code to the default initialization path to ensure the
definition is both present and valid.

Doug, please review. I think this follows your latest suggestion, and it ended
up remarkably cleaner than I anticipated. Also let me know if similar logic
should be followed for destructors and copy-constructors.

llvm-svn: 111802