DWARFv5 Emit DW_TAG_atomic_type for C11 _Atomic.

DWARFv5 Emit DW_TAG_atomic_type for C11 _Atomic.

This implements the DW_TAG_atomic_type for C11 _Atomic proposal as adopted
in the latest DWARF5 draft. http://dwarfstd.org/ShowIssue.php?issue=131112.1

gcc/ChangeLog

	PR debug/60782
	* dwarf2out.c (modified_type_die): Handle TYPE_QUAL_ATOMIC.

gcc/testsuite/ChangeLog

	PR debug/60782
	* gcc.dg/debug/dwarf2/atomic.c: New test.
	* gcc.dg/debug/dwarf2/stacked-qualified-types-3.c: Likewise.

include/ChangeLog

	PR debug/60782
	* dwarf2.def: Add DWARFv5 DW_TAG_atomic_type.

From-SVN: r218496