Debug info for blocks: Fix a bug caught by the Verifier.

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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222487 91177308-0d34-0410-b5e6-96231b3b80d8