c++: Don't crash when mangling member with anonymous union or template type [PR100632...

c++: Don't crash when mangling member with anonymous union or template type [PR100632, PR109790]

We currently crash upon mangling members that have an anonymous union or
a template operator type.

The problem is that before calling write_unqualified_name,
write_member_name asserts that it has a declaration whose DECL_NAME is
an identifier node that is not that of an operator. This is wrong:
 - In PR100632, it's an anonymous union declaration, hence a 0 DECL_NAME
 - In PR109790, it's a legitimate template declaration for an operator
   (this was accepted up to GCC 10)

This assert was added via r11-6301, to be sure that we do write the "on"
marker for operator members.

This patch removes that assert and instead
 - Lets members with an anonymous union type go through
 - For operators, adds the missing "on" marker for ABI versions greater
   than the highest usable with GCC 10

	PR c++/109790
	PR c++/100632

gcc/cp/ChangeLog:

	* mangle.cc (write_member_name): Handle members whose type is an
	anonymous union member. Write missing "on" marker for operators
	when ABI version is at least 16.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/decltype83.C: New test.
	* g++.dg/cpp0x/decltype83a.C: New test.
	* g++.dg/cpp1y/lambda-ice3.C: New test.
	* g++.dg/cpp1y/lambda-ice3a.C: New test.
	* g++.dg/cpp2a/nontype-class67.C: New test.