[Debug Info] replace DIUnspecifiedParameter with DITrivialType.

This is the first of a series of patches to handle type uniqueing of the
type array for a subroutine type.

This commit makes sure unspecified_parameter is a DIType to enable converting
the type array for a subroutine type to an array of DITypes.

This commit should have no functionality change. With this commit, we may
change unspecified type to be a DITrivialType instead of a DIType.