d: Remove d_size_t from front-end sources (PR 87788)

d: Remove d_size_t from front-end sources (PR 87788)

The typedef for d_size_t assumes that the implementation of the
front-end is written in D itself, where size_t can map only to uint32_t
or uint64_t.  As that is not the case for the current D front-end, the
typedef should be removed. This would fix the bootstrap on targets where
in C++ size_t is a long, such as darwin i386.

Reviewed-on: https://github.com/dlang/dmd/pull/12008

gcc/d/ChangeLog:

	PR d/87788
	* dmd/MERGE: Merge upsream dmd 45fa6cfd2.