c++: ICE when mangling operator name [PR98545]

c++: ICE when mangling operator name [PR98545]

r11-6301 added some asserts in mangle.c, and now we trip over one of
them.  In particular, it's the one asserting that we didn't get
IDENTIFIER_ANY_OP_P when mangling an expression with a dependent name.

As this testcase shows, it's possible to get that, so turn the assert
into an if and write "on".  That changes the mangling in the following
way:

With this patch:

$ c++filt _ZN1i1hIJ1adS1_EEEDTcldtdefpTonclspcvT__EEEDpS2_
decltype (((*this).(operator()))((a)(), (double)(), (a)())) i::h<a, double, a>(a, double, a)

G++10:
$ c++filt _ZN1i1hIJ1adS1_EEEDTcldtdefpTclspcvT__EEEDpS2_
decltype (((*this).(operator()))((a)(), (double)(), (a)())) i::h<a, double, a>(a, double, a)

clang++/icc:
$ c++filt _ZN1i1hIJ1adS1_EEEDTclonclspcvT__EEEDpS2_
decltype ((operator())((a)(), (double)(), (a)())) i::h<a, double, a>(a, double, a)

This is now tracked in PR98756.

gcc/cp/ChangeLog:

	PR c++/98545
	* mangle.c (write_member_name): Emit abi_warn_or_compat_version_crosses
	warnings regardless of abi_version_at_least.
	(write_expression): When the expression is a dependent name
	and an operator name, write "on" before writing its name.

gcc/ChangeLog:

	PR c++/98545
	* doc/invoke.texi: Update C++ ABI Version 15 description.

gcc/testsuite/ChangeLog:

	PR c++/98545
	* g++.dg/abi/mangle76.C: New test.