d: Merge upstream dmd 56589f0f4, druntime 651389b5, phobos 1516ecad9.

D front-end changes:

    - Import latest bug fixes to mainline.

D runtime changes:

    - Import latest bug fixes to mainline.

Phobos changes:

    - Import latest bug fixes to mainline.

gcc/d/ChangeLog:

	* dmd/MERGE: Merge upstream dmd 56589f0f4.

libphobos/ChangeLog:

	* libdruntime/MERGE: Merge upstream druntime 651389b5.
	* src/MERGE: Merge upstream phobos 1516ecad9.