[C++ PATCH] more TU parsing refactoring

[C++ PATCH] more TU parsing refactoring

https://gcc.gnu.org/ml/gcc-patches/2018-10/msg00788.html
	* parser.h (struct cp_parser): Drop implicit_extern_c.
	* parser.c (cp_debug_parser): Drop implicit_extern_c.
	(cp_parser_new): Likewise.
	(cp_parser_translation_unit): Handle implicit extern c here.  Call
	cp_parser_toplevel_declaration.
	(cp_parser_toplevel_declaration): New, broken out of ...
	(cp_parser_declaration_seq_opt): ... here.  Call it.  Drop
	implicit extern C handling.

From-SVN: r265127