PR analyzer/93993 reports another ICE within
diagnostic_manager::prune_for_sm_diagnostic in which the expression
of interest becomes a non-lvalue (similar to PR 93544, PR 93647, and
PR 93950), due to attempting to get an lvalue for a non-lvalue with a
NULL context, leading to an ICE when the failure is reported to
make_region_for_unexpected_tree_code.  The tree in question is
an ADDR_EXPR of a VAR_DECL, due to:
  event 11: switching var of interest from ‘tm’ in callee to ‘&qb’ in caller

This patch adds more bulletproofing to the routine by introducing
a tentative_region_model_context class that can be passed in such
circumstances which records that an error occurred, and then
checking to see if an error was recorded, thus avoiding the ICE.
This is papering over the problem, but a better solution seems more
like stage 1 material.

The patch also refactors the error-checking for CONSTANT_CLASS_P.

The testcase pr93993.f90 has a false positive:

 pr93993.f90:19:0:

    19 |     allocate (tm) ! { dg-warning "dereference of possibly-NULL" }
       |
 Warning: dereference of possibly-NULL ‘_6’ [CWE-690] [-Wanalyzer-possible-null-dereference]

which appears to be a pre-existing bug affecting any allocate call in
Fortran, which I will fix in a followup.

gcc/analyzer/ChangeLog:
	PR analyzer/93993
	* checker-path.h (state_change_event::get_lvalue): Add ctxt param
	and pass it to region_model::get_value call.
	* diagnostic-manager.cc (get_any_origin): Pass a
	tentative_region_model_context to the calls to get_lvalue and reject
	the comparison if errors occur.
	(can_be_expr_of_interest_p): New function.
	(diagnostic_manager::prune_for_sm_diagnostic): Replace checks for
	CONSTANT_CLASS_P with calls to update_for_unsuitable_sm_exprs.
	Pass a tentative_region_model_context to the calls to
	state_change_event::get_lvalue and reject the comparison if errors
	occur.
	(diagnostic_manager::update_for_unsuitable_sm_exprs): New.
	* diagnostic-manager.h
	(diagnostic_manager::update_for_unsuitable_sm_exprs): New decl.
	* region-model.h (class tentative_region_model_context): New class.

gcc/testsuite/ChangeLog:
	PR analyzer/93993
	* gfortran.dg/analyzer/pr93993.f90: New test.