[Clang] Fix BZ47169, loader_uninitialized on incomplete types

[Clang] Fix BZ47169, loader_uninitialized on incomplete types

Reported by @erichkeane. Fix proposed by @erichkeane works, tests included.
Bug introduced in D74361. Crash was on querying a CXXRecordDecl for
hasTrivialDefaultConstructor on an incomplete type. Fixed by calling
RequireCompleteType in the right place.

Reviewed By: erichkeane

Differential Revision: https://reviews.llvm.org/D85990