Fix issue 20929 - std.experimental.allocator.expandArray's range-based overload fails to instantiate for char and wchar arrays.

[just-harry]

https://issues.dlang.org/show_bug.cgi?id=20929

The instantiation failure was caused by trying to get the address of an lvalue
for emplace, which was caused by front's auto-decoding of the slice used to
refer to the uninitialised elements of the array.

The fix was to forgo the range functions for the slice, and just index it.

The changes to ForcedInputRange may seem unrelated to the fix, but I wanted
to test the input-range code-path, and the only usage of ForcedInputRange
was to test this overload of expandArray, so it seemed more appropriate to
reuse it, rather than define another input-range.

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20929	minor	std.experimental.allocator.expandArray's range-based overload fails to instantiate for char and wchar arrays.

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7530"