driver/c++: add --compile-std-module

driver/c++: add --compile-std-module

For simple testcases that want to use the std module, it would be useful to
have a reasonably short way to request building the binary module form
before the testcase.  So with this patch users can write

  g++ -std=c++20 -fmodules --compile-std-module mytest.C

I expect this to be particularly useful on godbolt.org, where currently
building a modules testcase requires messing with cmake.  One complication
there is that just adding std.cc to the command line arguments hits the
"cannot specify -o with -S with multiple files" error, so I avoid counting
these added inputs as "multiple files"; in that situation each compile will
output to the same target file, with the user-specified input last so it's
the one that actually ends up in the target after the command completes.

gcc/c-family/ChangeLog:

	* c.opt: Add --compile-std-module.

gcc/cp/ChangeLog:

	* lang-specs.h: Add @c++-system-module.
	* g++spec.cc (lang_specific_driver): Handle --compile-std-module.

gcc/ChangeLog:

	* doc/invoke.texi: Document --compile-std-module.
	* gcc.cc (struct infile): Add artificial field.
	(add_infile): Set it.
	(driver::prepare_infiles): Check it.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/compile-std1.C: New test.
	* g++.dg/modules/modules.exp: Only run it once.