Use Sema::RequireCompleteType to check for the completeness of

Use Sema::RequireCompleteType to check for the completeness of
Objective-C classes. This has two purposes: to consistently provide
"forward declaration here" notes when we hit an incomplete type, and
to give LLDB a chance to complete the type.

RequireCompleteType bits from Sean Callanan!

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@144573 91177308-0d34-0410-b5e6-96231b3b80d8