Debug info for blocks: Fix a bug caught by the Verifier.
When emitting nested block definitions, the insert-at-point variant of
DIBuilder::insertDeclare() could be called with the insertion point set
to the end-of-BasicBlock sentinel, causing the parent pointer of the
CallInst to be set to the intentionally bogus value of the sentinel.

Fixed by conditionally invoking the correct version of insertDeclare().
rdar://problem/19034882