[PR55641] drop spurious const_type from reference_type variables

[PR55641] drop spurious const_type from reference_type variables

Although C++ reference types, denoted by DW_TAG_reference_type in
DWARFv2+ debug info, are unchangeable, we output names of reference type
with DW_TAG_const_type, because internally we mark such variables as
TREE_READONLY.  That's an internal implementation detail that shouldn't
leak to debug information.  This patch fixes this.

for  gcc/ChangeLog

	PR debug/55641
	* dwarf2out.c (decl_quals): Don't map TREE_READONLY to
	TYPE_QUAL_CONST in reference-typed decls.

for  gcc/testsuite/ChangeLog

	PR debug/55641
	* g++.dg/debug/dwarf2/ref-1.C: New.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@239402 138bc75d-0d04-0410-961f-82ee72b054a4