Sema: Allow aliases to have incomplete type

gcc supports this behavior and it is pervasively used inside the Linux
kernel.

Note that both gcc and clang will reject code that attempts to do this
in a C++ language mode.

This fixes PR17998.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212631 91177308-0d34-0410-b5e6-96231b3b80d8