d: foreach over a tuple doesn't work on 16-bit targets (PR100999)

Improves semantic passes in the front-end around the `foreach' and
`static foreach' statements to be more resilient to compiling in a
minimal D runtime environment.  Checking of the index type has been
improved as well so now there won't be needless compiler errors when
using 8 or 16-bit integers as index types when the size fits the
expected loop range.

gcc/d/ChangeLog:

	PR d/100999
	* dmd/MERGE: Merge upstream dmd 7a3808254.

libphobos/ChangeLog:

	PR d/100999
	* src/MERGE: Merge upstream phobos 55bb17543.