diagnostics: add HTML output format as a plugin [PR116792]

diagnostics: add HTML output format as a plugin [PR116792]

This patch adds an experimental diagnostics output format that
writes HTML.  It isn't ready yet for end-users, but seems worth
keeping in the tree as I refactor the diagnostics subsystem, to
ensure that this code still builds, and to verify that it's possible to
implement new diagnostic output formats via GCC plugins. Hence
this patch merely adds it to the testsuite as an example of a GCC
plugin, rather than exposing it as a feature for end-users.

gcc/testsuite/ChangeLog:
	PR other/116792
	* gcc.dg/plugin/diagnostic-test-xhtml-1.c: New test.
	* gcc.dg/plugin/diagnostic_plugin_xhtml_format.c: New test plugin.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Add the above.

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