CGDebugInfo: Use DIImportedEntity default constructor, NFC

Use the newly minted `DIImportedEntity` default constructor (r230609)
rather than explicitly specifying `nullptr`.  The latter will become
ambiguous when the new debug info hierarchy is committed, since we'll
have both of the following:

    explicit DIImportedEntity(const MDNode *);
    DIImportedEntity(const MDImportedEntity *);

(Currently we just have the former.)

A default constructor is just as clear.

llvm-svn: 230610