c++: Strengthen checks on 'main'

c++: Strengthen checks on 'main'

This patch adds some missing requirements for legal main declarations,
as according to [basic.start.main] p2.

gcc/cp/ChangeLog:

	* decl.cc (grokfndecl): Check for main functions with language
	linkage or module attachment.
	(grokvardecl): Check for extern 'C' entities named main.

gcc/testsuite/ChangeLog:

	* g++.dg/abi/main.C: Check pedwarn for main with linkage-spec.
	* g++.dg/modules/contracts-1_b.C: Don't declare main in named
	module.
	* g++.dg/modules/contracts-3_b.C: Likewise.
	* g++.dg/modules/contracts-4_d.C: Likewise.
	* g++.dg/modules/horcrux-1_a.C: Export declarations, so that...
	* g++.dg/modules/horcrux-1_b.C: Don't declare main in named
	module.
	* g++.dg/modules/main-1.C: New test.
	* g++.dg/parse/linkage5.C: New test.
	* g++.dg/parse/linkage6.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>