Fix is_trivially_constructible preconditions.

Fixes a crash in cases where the first argument was an incomplete type
or an uninstantiated template type.

<rdar://problem/14938471>

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190482 91177308-0d34-0410-b5e6-96231b3b80d8