c++: Fix -std=c++20 ICE on virtual method call [PR99132]

On the following testcase we ICE in C++20 mode during cp_get_callee_fndecl
-> constexpr evaluation.
It is only in C++20 mode on this testcase because virtual methods can't
be constexpr in C++17 and earlier and so potential_constant_expression_1
rejects it earlier.
And the ICE is caused by genericization changing the h PARM_DECL from having
B<C *> type to B<C *> & DECL_BY_REFERENCE and the constexpr evaluation
not being able to deal with that.
I think this just shows that we shouldn't do the constexpr evaluation during
genericization and later, and other spots e.g. during gimplification
also don't call cp_get_callee_fndecl but cp_get_callee_fndecl_nofold.
After all, cp_fold has already been run and it did the folding if there
was any opportunity to do so.  And furthermore, what that cp_genericize_r
spot does is check for any left-over immediate function calls (which can be
ATM just std::source_location::current() call) and immediate functions
outside of immediate functions can't have addresses leaked into the IL,
so it will be always a direct call anyway.  And immediate functions
themselves don't make it into genericization/gimplification.

2021-02-18  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99132
	* cp-gimplify.c (cp_genericize_r) <case CALL_EXPR>: Use
	cp_get_callee_fndecl_nofold instead of cp_get_callee_fndecl to check
	for immediate function calls.

	* g++.dg/cpp2a/constexpr-virtual18.C: New test.

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