diagnostics: improve support for nesting levels [PR116253]

diagnostics: improve support for nesting levels [PR116253]

This patch adds support to sarif-replay for "nestingLevel"
from "P3358R0 SARIF for Structured Diagnostics"
https://wg21.link/P3358R0

Doing so revealed a bug where libgdiagnostics was always
creating new location_t values (and thus also
diagnostic_physical_location instances), rather than reusing
existing location_t values, leading to excess source printing.
The patch also fixes this bug, adding a new flag to libgdiagnostics
for debugging physical locations, and exposing this in sarif-replay
via a new "-fdebug-physical-locations" maintainer option.

Finally, the patch adds test coverage for the HTML sink's output
of nested diagnostics (both from a GCC plugin, and from sarif-replay).

gcc/ChangeLog:
	PR diagnostics/116253
	* diagnostics/context.cc (context::set_nesting_level): New.
	* diagnostics/context.h (context::set_nesting_level): New decl.
	* doc/libgdiagnostics/topics/compatibility.rst
	(LIBGDIAGNOSTICS_ABI_5): New.
	* doc/libgdiagnostics/topics/physical-locations.rst
	(diagnostic_manager_set_debug_physical_locations): New.
	* libgdiagnostics++.h (manager::set_debug_physical_locations):
	New.
	* libgdiagnostics-private.h
	(private_diagnostic_set_nesting_level): New decl.
	* libgdiagnostics.cc (diagnostic_manager::diagnostic_manager):
	Initialize m_debug_physical_locations.
	(diagnostic_manager::new_location_from_file_and_line): Add debug
	printing.
	(diagnostic_manager::new_location_from_file_line_column):
	Likewise.
	(diagnostic_manager::new_location_from_range): Likewise.
	(diagnostic_manager::set_debug_physical_locations): New.
	(diagnostic_manager::ensure_linemap_for_file_and_line): Avoid
	redundant calls to linemap_add.
	(diagnostic_manager::new_location): Add debug printing.
	(diagnostic_manager::m_debug_physical_locations): New field.
	(diagnostic::diagnostic): Initialize m_nesting_level.
	(diagnostic::get_nesting_level): New accessor.
	(diagnostic::set_nesting_level): New.
	(diagnostic::m_nesting_level): New field.
	(diagnostic_manager::emit_va): Set and reset the nesting level
	of the context from that of the diagnostic.
	(diagnostic_manager_set_debug_physical_locations): New.
	(private_diagnostic_set_nesting_level): New.
	* libgdiagnostics.h
	(diagnostic_manager_set_debug_physical_locations): New decl.
	* libgdiagnostics.map (LIBGDIAGNOSTICS_ABI_5): New.
	* libsarifreplay.cc (sarif_replayer::handle_result_obj): Support
	the "nestingLevel" property.
	* libsarifreplay.h (replay_options::m_debug_physical_locations):
	New field.
	* sarif-replay.cc: Add -fdebug-physical-locations.

gcc/testsuite/ChangeLog:
	PR diagnostics/116253
	* gcc.dg/plugin/diagnostic-test-nesting-html.c: New test.
	* gcc.dg/plugin/diagnostic-test-nesting-html.py: New test script.
	* gcc.dg/plugin/plugin.exp: Add it.
	* libgdiagnostics.dg/test-multiple-lines.c: Update expected output
	to show fix-it hint.
	* sarif-replay.dg/2.1.0-valid/nested-diagnostics-1.sarif: New test.

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