Add -fdiagnostics-path-format=separate-events to -fdiagnostics-plain-output

Add -fdiagnostics-path-format=separate-events to -fdiagnostics-plain-output

The path-printing default of -fdiagnostics-path-format=inline-events
interacted poorly with -fdiagnostics-plain-output, so it makes most
sense to add -fdiagnostics-path-format=separate-events to
-fdiagnostics-plain-output.

Seen when adding an experimental analyzer plugin to gcc.dg/plugin.exp.

gcc/ChangeLog:
	* doc/invoke.texi (-fdiagnostics-plain-output): Add
	-fdiagnostics-path-format=separate-events to list of
	options injected by -fdiagnostics-plain-output.
	* opts-common.c (decode_cmdline_options_to_array): Likewise.

gcc/testsuite/ChangeLog:
	* g++.dg/analyzer/analyzer.exp (DEFAULT_CXXFLAGS): Remove
	-fdiagnostics-path-format=separate-events.
	* gcc.dg/analyzer/analyzer.exp (DEFAULT_CFLAGS): Likewise.
	* gcc.dg/plugin/diagnostic-path-format-default.c: Rename to...
	* gcc.dg/plugin/diagnostic-path-format-plain.c: ...this.  Remove
	dg-options directive.  Copy remainder of test from
	diagnostic-path-format-separate-events.c.
	* gcc.dg/plugin/diagnostic-test-paths-2.c: Add
	-fdiagnostics-path-format=inline-events to options.
	Fix expected output for location of conditional within "for" loop.
	* gcc.dg/plugin/plugin.exp (plugin_test_list): Update for
	renaming.
	* gfortran.dg/analyzer/analyzer.exp (DEFAULT_FFLAGS): Remove
	-fdiagnostics-path-format=separate-events.