[ASTimporter] Remove decl from lookup only if it has decl context

In the case of TypedefDecls we set the DeclContext after we imported it.
It turns out, it could lead to null pointer dereferences during the
cleanup part of a failed import.

This patch demonstrates this issue and fixes it by checking if the
DeclContext is available or not.

Reviewed By: shafik

Differential Revision: https://reviews.llvm.org/D102640