c: Disallow braces around C2x auto initializers

c: Disallow braces around C2x auto initializers

WG14 agreed at this week's meeting to remove support for braces around
auto scalar initializers, as incompatible with C++ auto handling of
braced initializers; thus remove that support in GCC.

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

gcc/c/
	* c-parser.cc (c_parser_declaration_or_fndef): Do not allow braces
	around auto initializer.

gcc/testsuite/
	* gcc.dg/c2x-auto-1.c, gcc.dg/c2x-auto-3.c: Expect braces around
	auto initializers to be disallowed.