analyzer: add warnings about writes to constant regions [PR95007]

This patch adds two new warnings:
  -Wanalyzer-write-to-const
  -Wanalyzer-write-to-string-literal
for code paths where the analyzer detects a write to a constant region.

As noted in the documentation part of the patch, the analyzer doesn't
prioritize detection of such writes, in that the state-merging logic
will blithely lose the distinction between const and non-const regions.
Hence false negatives are likely to arise due to state-merging.

However, if the analyzer does happen to spot such a write, it seems worth
reporting, hence this patch.

gcc/analyzer/ChangeLog:
	* analyzer.opt (Wanalyzer-write-to-const): New.
	(Wanalyzer-write-to-string-literal): New.
	* region-model-impl-calls.cc (region_model::impl_call_memcpy):
	Call check_for_writable_region.
	(region_model::impl_call_memset): Likewise.
	(region_model::impl_call_strcpy): Likewise.
	* region-model.cc (class write_to_const_diagnostic): New.
	(class write_to_string_literal_diagnostic): New.
	(region_model::check_for_writable_region): New.
	(region_model::set_value): Call check_for_writable_region.
	* region-model.h (region_model::check_for_writable_region): New
	decl.

gcc/ChangeLog:
	* doc/invoke.texi: Document -Wanalyzer-write-to-const and
	-Wanalyzer-write-to-string-literal.

gcc/testsuite/ChangeLog:
	PR c/83347
	PR middle-end/90404
	PR analyzer/95007
	* gcc.dg/analyzer/write-to-const-1.c: New test.
	* gcc.dg/analyzer/write-to-string-literal-1.c: New test.

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