analyzer: add notes to write-to-const/string from access attr [PR104793]

analyzer: add notes to write-to-const/string from access attr [PR104793]

The previous patch extended
  -Wanalyzer-write-to-const
  -Wanalyzer-write-to-string-literal
to make use of __attribute__ ((access, ....), but the results could be
inscrutable.

This patch adds notes to such diagnostics to give the user a reason for
why the analyzer is complaining.

Example output:

test.c: In function 'main':
test.c:15:13: warning: write to string literal [-Wanalyzer-write-to-string-literal]
   15 |         if (getrandom((char *)test, sizeof(buf), GRND_RANDOM))
      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  'main': event 1
    |
    |   15 |         if (getrandom((char *)test, sizeof(buf), GRND_RANDOM))
    |      |             ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    |      |             |
    |      |             (1) write to string literal here
    |
test.c:3:5: note: parameter 1 of 'getrandom' marked with attribute 'access (write_only, 1, 2)'
    3 | int getrandom (void *__buffer, size_t __length,
      |     ^~~~~~~~~

Unfortunately we don't have location information for the attributes
themselves, just the function declaration, and there doesn't seem to be
a good way of getting at the location of the individual parameters from
the middle end (the C and C++ FEs both have get_fndecl_argument_location,
but the implementations are different).

gcc/analyzer/ChangeLog:
	PR analyzer/104793
	* analyzer.h (class pending_note): New forward decl.
	* diagnostic-manager.cc (saved_diagnostic::saved_diagnostic):
	Initialize m_notes.
	(saved_diagnostic::operator==): Compare m_notes.
	(saved_diagnostic::add_note): New.
	(saved_diagnostic::emit_any_notes): New.
	(diagnostic_manager::add_note): New.
	(diagnostic_manager::emit_saved_diagnostic): Call emit_any_notes
	after emitting the warning.
	* diagnostic-manager.h (saved_diagnostic::add_note): New decl.
	(saved_diagnostic::emit_any_notes): New decl.
	(saved_diagnostic::m_notes): New field.
	(diagnostic_manager::add_note): New decl.
	* engine.cc (impl_region_model_context::add_note): New.
	* exploded-graph.h (impl_region_model_context::add_note): New
	decl.
	* pending-diagnostic.h (class pending_note): New.
	(class pending_note_subclass): New template.
	* region-model.cc (class reason_attr_access): New.
	(check_external_function_for_access_attr): Add class
	annotating_ctxt and use it when checking region.
	(noop_region_model_context::add_note): New.
	* region-model.h (region_model_context::add_note): New vfunc.
	(noop_region_model_context::add_note): New decl.
	(class region_model_context_decorator): New.
	(class note_adding_context): New.

gcc/testsuite/ChangeLog:
	PR analyzer/104793
	* gcc.dg/analyzer/write-to-const-2.c: Add dg-message directives
	for expected notes.
	* gcc.dg/analyzer/write-to-function-1.c: Likewise.
	* gcc.dg/analyzer/write-to-string-literal-2.c: Likewise.
	* gcc.dg/analyzer/write-to-string-literal-3.c: Likewise.
	* gcc.dg/analyzer/write-to-string-literal-4.c: Likewise.
	* gcc.dg/analyzer/write-to-string-literal-5.c: New test.

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