analyzer: fix ICE on pointer arithmetic with incomplete types [PR 93774]

analyzer: fix ICE on pointer arithmetic with incomplete types [PR 93774]

PR analyzer/93774 reports an ICE in gfortran with -fanalyzer within
region_model::convert_byte_offset_to_array_index on a pointer of
incomplete type ("character(kind=1)[0:][1:0] * restrict").

This patch bulletproofs the routine against incomplete types, fixing
the ICE.

gcc/analyzer/ChangeLog:
	PR analyzer/93774
	* region-model.cc
	(region_model::convert_byte_offset_to_array_index): Use
	int_size_in_bytes before calling size_in_bytes, to gracefully fail
	on incomplete types.

gcc/testsuite/ChangeLog:
	PR analyzer/93774
	* gfortran.dg/analyzer/deferred_character_25.f90: New test,
	based on gfortran.dg/deferred_character_25.f90.