C++: show location of unclosed extern "C" specifications

C++: show location of unclosed extern "C" specifications

If the user fails to close an extern "C" linkage specifier, and then
uses templates, they will run into "template with C linkage" errors.

From personal experience, it can be hard to tell where the
extern "C" began.  As of r251026 there will be a message highlighting
the unclosed '{', but this may be hard to spot at the very end of
the errors.

This patch adds a note to the various diagnostics that complain
about C linkage, showing the user where the extern "C" specification
began.

gcc/cp/ChangeLog:
	* cp-tree.h (maybe_show_extern_c_location): New decl.
	* decl.c (grokfndecl): When complaining about literal operators
	with C linkage, issue a note giving the location of the
	extern "C".
	* parser.c (cp_parser_new): Initialize new field
	"innermost_linkage_specification_location".
	(cp_parser_linkage_specification): Store the location
	of the linkage specification within the cp_parser.
	(cp_parser_explicit_specialization): When complaining about
	template specializations with C linkage, issue a note giving the
	location of the extern "C".
	(cp_parser_explicit_template_declaration): Likewise for templates.
	(maybe_show_extern_c_location): New function.
	* parser.h (struct cp_parser): New field
	"innermost_linkage_specification_location".

gcc/testsuite/ChangeLog:
	* g++.dg/cpp0x/udlit-extern-c.C: New test case.
	* g++.dg/diagnostic/unclosed-extern-c.C: Add example of a template
	erroneously covered by an unclosed extern "C".
	* g++.dg/template/extern-c.C: New test case.

From-SVN: r253726