ISO C17 won't go to ballot until December, meaning publication of the
standard won't be until 2018, leaving ambiguity as to whether people
will end up referring to the standard as C17, as it's currently known
and which corresponds to the __STDC_VERSION__ value, or C18 based on
the publication date.

In case people end up referring to this standard as C18, this patch
adds corresponding option aliases -std=c18, -std=iso9899:2018,
-std=gnu18 so people can use those names based on publication date if
they wish.  The "expected to be" explanations in help texts and the
manual can be removed as and when the standard is published, hopefully
before GCC 8 is out.

Bootstrapped with no regressions on x86_64-pc-linux-gnu.

gcc:
	* doc/invoke.texi (-std=c17): Refer to 2018 expected publication
	date of C17.
	(-std=c18, -std=iso9899:2018, -std=gnu18): Document option
	aliases.

gcc/c-family:
	* c.opt (-std=c17, std=gnu17, -std=iso9899:2017): Refer to 2018
	expected publication date of C17.
	(-std=c18, -std=gnu18, -std=iso9899:2018): New option aliases.

gcc/testsuite:
	* gcc.dg/c18-version-1.c, gcc.dg/c18-version-2.c: New tests.

From-SVN: r254847