analyzer: fix ICE adding note to disabled diagnostic [PR104997]

analyzer: fix ICE adding note to disabled diagnostic [PR104997]

gcc/analyzer/ChangeLog:
	PR analyzer/104997
	* diagnostic-manager.cc (diagnostic_manager::add_diagnostic):
	Convert return type from "void" to "bool", reporting success vs
	failure to caller, for both overloads.
	* diagnostic-manager.h (diagnostic_manager::add_diagnostic):
	Likewise.
	* engine.cc (impl_region_model_context::warn): Propagate return
	value from diagnostic_manager::add_diagnostic.

gcc/testsuite/ChangeLog:
	PR analyzer/104997
	* gcc.dg/analyzer/write-to-string-literal-4-disabled.c: New test,
	adapted from write-to-string-literal-4.c.

Signed-off-by: David Malcolm <dmalcolm@redhat.com>