d: Merge upstream dmd cef1e7991.

d: Merge upstream dmd cef1e7991.

Adds a DString type, a struct that has a compatible layout with D
strings.  Many parameters in the Global struct have been switched over
to this type, and users of these params have been adjust to use the
length or ptr field as appropriate.

gcc/d/ChangeLog:

	* dmd/MERGE: Merge upstream dmd cef1e7991.
	* d-lang.cc (d_parse_file): Adjust for new field types.