Use RequireCompleteType() instead of isIncompleteType().

isIncompleteType() returns true or false for template types depending on whether
the type is instantiated yet. In this context, that's arbitrary. The better way
to check for a complete type is RequireCompleteType().

Thanks to Eli Friedman for noticing this!

<rdar://problem/12700799>


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@177768 91177308-0d34-0410-b5e6-96231b3b80d8