Add GCC_EXTRA_DIAGNOSTIC_OUTPUT environment variable for fix-it hints

Add GCC_EXTRA_DIAGNOSTIC_OUTPUT environment variable for fix-it hints

GCC has had the ability to emit fix-it hints in machine-readable form
since GCC 7 via -fdiagnostics-parseable-fixits and
-fdiagnostics-generate-patch.

The former emits additional specially-formatted lines to stderr; the
option and its format were directly taken from a pre-existing option
in clang.

Ideally this could be used by IDEs so that the user can select specific
fix-it hints and have the IDE apply them to the user's source code
(perhaps turning them into clickable elements, perhaps with an
"Apply All" option, etc).  Eclipse CDT has supported this option in
this way for a few years:
  https://bugs.eclipse.org/bugs/show_bug.cgi?id=497670

As a user of Emacs I would like Emacs to support such a feature.
https://debbugs.gnu.org/cgi/bugreport.cgi?bug=25987 tracks supporting
GCC fix-it output in Emacs.  The discussion there identifies two issues
with the existing option:

(a) columns in the output are specified as byte-offsets within the
line (for exact compatibility with the option in clang), whereas emacs
would prefer to consume them as what GCC 11 calls "display columns".
https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Message-Formatting-Options.html#index-fdiagnostics-column-unit

(b) injecting a command-line option into the build is a fiddly manual
step, varying between build systems.  It's far easier for the
user if Emacs simply sets an environment variable when compiling,
GCC uses this to enable the option if it recognizes the value, and
the emacs compilation buffer decodes the additional lines of output
and adds appropriate widgets.  In some ways it is a workaround for
not having a language server.  Doing it this way means that for the
various combinations of older and newer GCC and older and newer Emacs
that a sufficiently modern combination of both can automatically
support the rich fix-it UI, whereas other combinations will either
not provide the envvar, or silently ignore it, gracefully doing
nothing extra.

Hence this patch adds a new GCC_EXTRA_DIAGNOSTIC_OUTPUT environment
variable to GCC which enables output of machine-parseable fix-it hints.

GCC_EXTRA_DIAGNOSTIC_OUTPUT=fixits-v1 is equivalent to the existing
-fdiagnostics-parseable-fixits option.

GCC_EXTRA_DIAGNOSTIC_OUTPUT=fixits-v2 is the same, but changes the
column output mode to "display columns" rather than bytes, as
required by Emacs.

The discussion in that Emacs bug has some concerns about the encoding
of these lines, and, indeed, the encoding of GCC's stderr in general:
currently we emit a mixture of bytes and UTF-8; I believe we emit
filenames as bytes, diagnostic messages as UTF-8, and quote source code
in the original encoding (PR other/93067 covers converting it to UTF-8 on
output).  This patch prints octal-escaped bytes for bytes within
filenames and replacement text that aren't printable (as per
-fdiagnostics-parseable-fixits).

gcc/ChangeLog:
	* diagnostic.c (diagnostic_initialize): Eliminate
	parseable_fixits_p in favor of initializing extra_output_kind from
	GCC_EXTRA_DIAGNOSTIC_OUTPUT.
	(convert_column_unit): New function, split out from...
	(diagnostic_converted_column): ...this.
	(print_parseable_fixits): Add "column_unit" and "tabstop" params.
	Use them to call convert_column_unit on the column values.
	(diagnostic_report_diagnostic): Eliminate conditional on
	parseable_fixits_p in favor of a switch statement on
	extra_output_kind, passing the appropriate values to the new
	params of print_parseable_fixits.
	(selftest::test_print_parseable_fixits_none): Update for new
	params of print_parseable_fixits.
	(selftest::test_print_parseable_fixits_insert): Likewise.
	(selftest::test_print_parseable_fixits_remove): Likewise.
	(selftest::test_print_parseable_fixits_replace): Likewise.
	(selftest::test_print_parseable_fixits_bytes_vs_display_columns):
	New.
	(selftest::diagnostic_c_tests): Call it.
	* diagnostic.h (enum diagnostics_extra_output_kind): New.
	(diagnostic_context::parseable_fixits_p): Delete field in favor
	of...
	(diagnostic_context::extra_output_kind): ...this new field.
	* doc/invoke.texi (Environment Variables): Add
	GCC_EXTRA_DIAGNOSTIC_OUTPUT.
	* opts.c (common_handle_option): Update handling of
	OPT_fdiagnostics_parseable_fixits for change to diagnostic_context
	fields.

gcc/testsuite/ChangeLog:
	* gcc.dg/plugin/diagnostic-test-show-locus-GCC_EXTRA_DIAGNOSTIC_OUTPUT-fixits-v1.c:
	New file.
	* gcc.dg/plugin/diagnostic-test-show-locus-GCC_EXTRA_DIAGNOSTIC_OUTPUT-fixits-v2.c:
	New file.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add them.