analyzer: fix missing -Wanalyzer-write-to-const [PR102695]

analyzer: fix missing -Wanalyzer-write-to-const [PR102695]

This patch fixes -Wanalyzer-write-to-const so that it will complain
about attempts to write to functions, to labels.
It also "teaches" the analyzer about strchr, in that strchr can either
return a pointer into the input area (and thus -Wanalyzer-write-to-const
can now complain about writes into a string literal seen this way),
or return NULL (and thus the analyzer can complain about NULL
dereferences if the result is used without a check).

gcc/analyzer/ChangeLog:
	PR analyzer/102695
	* region-model-impl-calls.cc (region_model::impl_call_strchr): New.
	* region-model-manager.cc
	(region_model_manager::maybe_fold_unaryop): Simplify cast to
	pointer type of an existing pointer to a region.
	* region-model.cc (region_model::on_call_pre): Handle
	BUILT_IN_STRCHR and "strchr".
	(write_to_const_diagnostic::emit): Add auto_diagnostic_group.  Add
	alternate wordings for functions and labels.
	(write_to_const_diagnostic::describe_final_event): Add alternate
	wordings for functions and labels.
	(region_model::check_for_writable_region): Handle RK_FUNCTION and
	RK_LABEL.
	* region-model.h (region_model::impl_call_strchr): New decl.

gcc/testsuite/ChangeLog:
	PR analyzer/102695
	* gcc.dg/analyzer/pr102695.c: New test.
	* gcc.dg/analyzer/strchr-1.c: New test.

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