c: Allow omitted parameter names for C2x

c: Allow omitted parameter names for C2x

C2x allows parameter names to be omitted in function definitions, as
in C++; add support for this feature.  As with other features that
only result in previously rejected code being accepted, this feature
is now accepted as an extension for previous standard versions, with a
pedwarn-if-pedantic that is disabled by -Wno-c11-c2x-compat.  The
logic for avoiding unused-parameter warnings for unnamed parameters is
in code shared between C and C++, so no changes are needed there.

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

gcc/c/
2020-10-28  Joseph Myers  <joseph@codesourcery.com>

	* c-decl.c (store_parm_decls_newstyle): Use pedwarn_c11 not
	error_at for omitted parameter name.

gcc/testsuite/
2020-10-28  Joseph Myers  <joseph@codesourcery.com>

	* gcc.dg/c11-parm-omit-1.c, gcc.dg/c11-parm-omit-2.c,
	gcc.dg/c11-parm-omit-3.c, gcc.dg/c11-parm-omit-4.c,
	gcc.dg/c2x-parm-omit-1.c, gcc.dg/c2x-parm-omit-2.c,
	gcc.dg/c2x-parm-omit-3.c, gcc.dg/c2x-parm-omit-4.c: New tests.
	* gcc.dg/noncompile/pr79758.c: Do not expect error for omitted
	parameter name.