c++, debug: Treat -std=c++20 -gdwarf-5 like C++14 rather than C++98

c++, debug: Treat -std=c++20 -gdwarf-5 like C++14 rather than C++98

I have noticed that while we use DW_LANG_C_plus_plus_14 for -std=c++17 -gdwarf-5,
we use DW_LANG_C_plus_plus (aka C++98) for -std=c++20 -gdwarf-5.  The
following patch makes those two match.

2020-12-01  Jakub Jelinek  <jakub@redhat.com>

	* dwarf2out.c (gen_compile_unit_die): Treat GNU C++20
	like C++14 for -gdwarf-5.

	* g++.dg/debug/dwarf2/lang-cpp17.C: New test.
	* g++.dg/debug/dwarf2/lang-cpp20.C: New test.