When we import the definition for a Tagdecl, propagate its completeness too.

The ASTImporter completes the full definiton for a TagDecl in several places,
including the type-deport logic.  When this happens, we should also propagate
the bit that says that this is a complete definition.  This makes (for example)
lambdas callable.

<rdar://problem/22864976>

llvm-svn: 264485