DWARF add DW_AT_noreturn on noreturn function subprogram.

DWARF add DW_AT_noreturn on noreturn function subprogram.

This implements the DWARFv5 noreturn proposal:
http://dwarfstd.org/ShowIssue.php?issue=140331.1

TREE_THIS_VOLATILE on a FUNCTION_DECL node means the function does not
return normally. This catches the traditional noreturn GNU attribute,
the C11 _Noreturn keyword and the C++11 [[noreturn]] attribute.

This relies on the DW_AT_noreturn constant defined in the DWARFv5 DRAFT:
http://www.dwarfstd.org/doc/dwarf5.20141029.pdf

gcc/ChangeLog

	* dwarf2out.c (gen_subprogram_die): Add DW_AT_noreturn when the
	function decl has TREE_THIS_VOLATILE.

gcc/testsuite/ChangeLog

	* g++.dg/debug/dwarf2/noreturn-function.C: New test.
	* gcc.dg/debug/dwarf2/noreturn-function-attribute.c: Likewise.
	* gcc.dg/debug/dwarf2/noreturn-function-keyword.c: Likewise.

include/ChangeLog

	* dwarf2.def (DW_AT_noreturn): New DWARF5 attribute.

From-SVN: r218075