analyzer: fix ICE with fortran constant arguments (PR 93405)

analyzer: fix ICE with fortran constant arguments (PR 93405)

PR analyzer/93405 reports an ICE with -fanalyzer when passing
a constant "by reference" in gfortran.

The issue is that the constant is passed as an ADDR_EXPR
of a CONST_DECL, and region_model::get_lvalue_1 doesn't
know how to handle CONST_DECL.

This patch implements it for CONST_DECL by providing
a placeholder region, holding the CONST_DECL's value,
fixing the ICE.

gcc/analyzer/ChangeLog:
	PR analyzer/93405
	* region-model.cc (region_model::get_lvalue_1): Implement
	CONST_DECL.

gcc/testsuite/ChangeLog:
	PR analyzer/93405
	* gfortran.dg/analyzer/pr93405.f90: New test.